Yes, they are very comfortable to 'sit' on as they support the body very well.
 
J. Tanner and I have had many discussions about how a recumbent seat should be. He's a LMT and works for a group of doctors in that capacity so has a huge amount of technical information on how the body should be supported. He's the only rider I know of who uses a lower seatback angle than I. About three or four years ago we realized we were trying to accomplish the same things but were using different means which were both very successful and we began discussing how a recumbent seat should really be shaped and have continued over the years. Every time I have him try the latest seat out he gives me excellent feedback on why what I have done works so well from a physiological standpoint. One of these days I likely ought to discuss that background. What we've found is pretty much the opposite of traditional recumbent seat design so far and when you take a close look, it's pretty obvious why. Tanner has gently tried to explain the principles to a certain recumbent company to no effect. Not that anyone is asking them to actually make any changes but maybe just to slow down the disinformation so people have the knowledge to make the right choice they need (whatever it is).

Covering a few bases very briefly (need more time to cover adequately).  
Several issues come into play, depending on what sort of riding you do. For short, fairly infrequent rides, you might not encounter issues and that seems to be the case for the large majority of recumbent riders. It's when you start riding more frequently and for longer periods of time that issues tend to develop. Those issues tend to be related to the sacrum and coccyx first and then may spread to spinal issues if the rider doesn't already have those.
 
Pressure on the coccyx (tailbone) can be very painful and can cause nerve damage fairly quickly. You may have noticed that most of us ultra riders use a lumbar support panel of some sort on conventional carbon seats. This is to allow the lower back to carry the weight and clear the tailbone from stress. While there are many ways to accomplish this, a higher seatback tends to concentrate pressure over a smaller area. As the seatback is lowered, that weight can be distributed over a much larger surface area, spreading further up your back.
 
If you take a look at diagrams, etc. of the sacrum, you might see the network of blood vessels and nerves that penetrate it to connect with your legs. Pressure on the sacrum can restrict blood flow to your legs and again, pinch and tend to inflame nerves that control and respond to leg muscles. Since the blood vessels transport fuel to the leg muscles it is critical they not be constricted. I can tell you that pinched or otherwise constricted nerves in that region can cause long-lasting problems that most certainly restrict power generation.
 
So what positions allow you to make power and especially maintain it?
 
People point to some writeups done a few years back claiming that the closed, more upright position allowed for greater power generation. I have no idea how those experiments were performed. As a research scientist for many years, I am fully aware of how important it is to properly set up and perform experiments and how badly the results can go astray when this is not done. I also know that experimental results can be taken out of context and accomplish the same thing. I'm not sure what the case here is but I do firmly believe and have shown with my own results (and others who have used my methods) that the closed position simply does not guarantee more power or better/faster climbing. Nor does the open, low seatback angle.  Success depends on getting things right! And that means you will need to spend some time investigating what is right for you and how to prevent possible coccyx and sacrum issues from inhibiting comfort and power production. Existing recumbent designs tend to not provide the sort of protection for the coccyx and sacrum that many (if not most) people need. You will need to find something that works. I will say that a few seats I've seen recently are starting to get closer and hopefully are beginning to recognize these issues.
 
Outside of comfort and power production there is simple wind resistance. If you can generate the same power in either the closed or open positions, which do you think will be faster? :^)
 
As to your question of support and the mods you have made, it sounds like you are on the right track. If you can arrange to support your pelvis so that it is easy to use it as a lever to slightly lessen the weight your glutes put on the seat, this will also improve bloodflow to the legs... Distribute the pressure out across the top to the sides of your pelvis so it is not concentrated, then you can lock your pelvis into the seat and generate your power from there. No need to bridge and bring the upper back muscles into play. Let them do their job of just supporting the frame. This relieves pressure from the shoulder blades, etc. also. When you get it right, you can use that little pelvic tilt to greatly extend your power generation over time. You might need to build up a few muscles to do it, but I think you will enjoy the results.

I've also noticed that the other thing that happens with recumbent seats is the effect of compression of the foam...  the "filter foam" of the Hoagie is typically compressed to one level under normal riding, but when you start pushing, it compresses further, which slightly changes your effective seat-to-pedal distance and leg extension... so you have to set your seat such that you're not overextended under power.  Would you say this is partially fixed by going to a denser pad?  Which then transmits more vibration, of course -- everything's a tradeoff...

Quote from Rat Rider on Dec 18^th, 2013, 10:43pm:

I built another RailGun seat for this guy for his other F5 Pro. He wanted to have a seat that could recline all of the way back on this bike so I had to build another custom rib. It is likely close to 10x stronger than the one I built for my M5 but is a little heavier also. I replaced the old F5 pic with the new seat and other F5. The pic shows the seat stays on the second to last hole but he ended up having to move the seat a half-inch further forward than he thought so he can move to the last hole and likely still clear the seatback. He is really laid back for such a big guy and yes, that's a fair amount of weight aft of the rear axle. Neat thing is the bulk of his weight has actually moved forward; about 2 1/2". Give him a little bit of time to get used to the position and I expect he'll be much more stable on it than before.
 
I also worked out a better (and far stronger) way of supporting the front seat mount on this seat. And yes, he loves the seatback flair. I still laugh at that as it never crossed my mind that the flair would be such an attraction. I did it to be able to bond the tailbox to the seat more efficiently and strongly. Every rider who has tried it out loves that feature.

Will do.  It may be a while.