I installed one of these backrests a few years ago...had the same experience. After a long trip, I came home and my arms/shoulders were in rough shape.

I don't have any photos of me modifying the seat pan, but it is really easy. As in any advice, my top recommendation is TAKE YOUR TIME! I think going as slow as I did, it took me about an hour to do the mod.

If I recall correctly, you need a longer knife, which I made use of a larger steak knife with a finer serrated blade, it wasn't that rough. I think there is a template for locating the cut in the seat where the bracket is punched through the seat. The knife is used to separate the foam internally in the seat to dislodge it so you can force the bracket up on the seat pan and punch it through the seat. There are something like four bolts that are easily used to attach the bracket to the bottom of the seat.

What I like about the backrest is that you can remove it easily, AND it looks like it is built for this seat...in other words, it doesn't look ugly on the bike. I made a quick disconnect for the bolt that holds the backrest in the bracket, just a few clever handles from a hardware store, I can easily remove it with bare hands without any problem. Also, the backrest can be adjusted fore/aft which may take a few adjustments to get just right, but it is almost infinite in range.

I am 5'8" and have the back rest almost all the way to the forward position, it pivots and is quite comfortable. I can now do 500 mile days that I could never do before on the bike. I think there is someone with photos of the back rest install. It really is not too difficult to get right, I highly recommend the backrest. I don't think the gunfighter seat would be the way to go compared to a true backrest like this.