Thanks for the tip, DocLizard, and feedback, SuperNintendo.
I checked with Spencer's Seat Mods and he sent me a very detailed email back with the intended modifications and required information. The price also seemed reasonable, but sending the seat back and forth and considering that I wanted to checkout my 919 before going on a long trip (1,600 miles one-way) meant that time-wise, things could get tight, especially if the seat got lost somewhere.
So - I went to a wholesale upholstery shop in Denver recommended by a local upholsterer and looked at their gel-pad options. It did not work for me as I want to feel it when the rear tire breaks loose. They showed me the foam used by many upholsterers in the Denver region and it seems like that one will do the job. I tried other ones, but they were either too soft or way too hard.
Another consideration is that I want to sit on the foam before the upholsterer puts the vinyl seat cover on.
The plan is to shave off one inch off the firm seat foam and replace it with a medium firm foam, which in theory should actually drop my seat height by, say, a half of an inch. So it should not raise me on the seat at all. I do want to bulge out the sides a bit (1/4" to 1/2") to give me more support. If it doesn't work, he can still taper it back before the seat cover is installed.
I will send an update with pictures once I have the seat back and checked out.