Wow! I picked one up from a fellow member here on the forum and this thing rocks! Night and day better than the Guts tall soft I had on my last bike. They aren't cheap but boy what a difference they make. Mine is about 5/8" taller and also 2-3 inches wider at the back. Makes the second half of our all-day rides a lot more fun.
Fisher is a custom aftermarket company. You send them your seat and $250 and they modify it and send it back. Seat Concepts is another fantastic seat but they no longer do custom work or kits for GasGas. Their kits are $160, I suppose you could buy a kit for a jap bike and make it work if the length was the same or just a little longer.
http://www.fishersaddlebags.com/seats.html
http://www.seatconcepts.com/products