I have another option to throw in the ring for those who feel the 2" mattress is too firm. The short version is Exped Megamat Duo. The long version follows. 
My wife and I spend a lot of nights sleeping outdoors each year. Probably in excess of a month total between backpacking, backcountry ski huts, and car camping. When we’re car camping we’ve decided we want to be as comfortable as possible and get the best sleep. The Exped Megamat is great. It rivals our home mattress in comfort. We particularly like that it is firm without being too hard, you don’t feel your partner move much, and it has great R-value. If you haven’t tried one, call your local REI and see if they have a demo in the store.
When we got the GFC we were willing to accept some backslide in mattress comfort for all the other benefits it provides. We first rented an early GFC unit to try-before-buying. That had the 3" mattress and if I recall right, it was comfy but my hips and shoulders bottomed out a bit. When we got our GFC a few months ago (#297), it had the 2" mattress. This bottoms out less but my wife and I both found it to be too firm. Like others here, we added a 1" latex topper to it and this works pretty well. It was soft enough and the GFC closes okay. But, I’m always looking to improve on things.
Before a recent round of trips, it occured to me to try the Megamat in the GFC. We have about 5 nights out with it now…and I think we’ll stick with the Exped because all of its good features come through in the GFC. It is super comfortable up there now. But, it isn’t without downsides. Here are the issues:
- Depth of the mattress: the Megamat is 3.9" think when inflated. So the GFC will not close until you deflate it (but that’s part of the point, it deflates a lot!).
- Width of the mattress: the Megamat is spec’d at 52" wide. About 2" wider than the GFC. In reality, it is closer to 51" and when inflated it conforms pretty well to the constrained width of the GFC. The tent fabric near the extrusion has a little extra pressure on it from the inside but I don’t anticipate any change in performance or dealing with precipitation. I’ll report back if there are issues in a big rain with water wicking in under this pressure. It also means GFC won’t close comfortably with the mattress laid fully out.
- Deflating the Megamat: Because of #1 and #2, the Megamat has to be deflated (for depth) and folded in half (for width) for the GFC to close. This adds about 5 minutes to my take down process but that’s okay because it was part of my old routine with the Megamat, it can be done inside the GFC and out of the elements, and the GFC closes really easily with sheets, a wool blanket, a down comforter and 2 pillows up there once I do this. The comfort is worth the extra time for me. To deflate, I set the pillows to one side, then fold the Megamat in half width-wise with sheets, blanket and comforter in place. I lay on top of it until it stops deflating, then I fold it in thirds length-wise, each time leaning on it until no more air comes out.
- Length: The Megamat is available in 72" and 77" lengths, I think. I have the 77". So it interferes with the transforma-floor. When you take either of the two small panels out it overlaps the opening by about 50%. Fortunately, the Exped is flexible and it is easy for me or my wife (who is 4.5 months pregnant) to slide through the opening. I don’t consider this a big issue but it might be for some people.
- Inflating: The Exped is self-inflating and this works great under most circumstances. But in the colder weather of fall, with the weight of blankets on top, …it needs some help. I do about 20 strokes on a low-pressure, high-volume pump (like for an inflatable raft) and it is ready to go. Inflation adds about 2 minutes to my setup time.
- Cost: The Megamat is not cheap. If I didn’t already have one for car camping - I might have stuck with the latex topper approach for now.
Sorry for the long post, but I think the Exped is a pretty interesting option for people looking for great comfort in the GFC and who don’t mind the downsides listed above.