TX, you are likely going to want to do a couple of other things as well. I personally have not done the mod you are asking about, but I have had occasion to remove and re-attach a portion of my cover. In my opinion, you could not add enough to make any difference and still use the same cover. So, if you want to add foam, think, new cover. NOW, you may be able to raise the stock seat by placing spacers of some sort under the two rear attachment points and fabbing an additional front snout to go under the frame bracket. As I say this, my mind has raced ahead of my fingers. Raising the entire seat would (my opinion) be inexpensive, reasonably quick and very doable.
Now the glitch....... Many of us tall guys have needed (it seems) to raise the bars, before even giving any thought to the seat. NOW, if you raise the seat, you will likely need to (want to) raise the bars even more. Bar raising is limited by control cable length.
I have not heard of anyone lowering the pegs. For off-pavement, the TW is fairly low as it is. If I may be so bold TX; give some thought to dealing with the bars first and then re-assess. Gerry
Be safe my Friend......