There has been some good discussion on this forum about seats, you might want to search it. One of the things you will probably find is a warning about making a seat too perfect a match, it will probable be uncomfortable after a short time.

I am a fan of the Thermarest touring seat. It is much thicker than the standard Pygmy handout and it has a contour to it. If conditions get dicey I can let out some air and lower my CG, or, add some air if I want lessen stability and hold a lean more easily. I use it in tandem with the SeaLine Kayak Thigh Support Cushion and have spendt many consecutive hours in the kayak without back or leg complaints. I have cronic sciatica so I am familiar the numb-pain you spoke of. BTW I found that recreational kayaks such as my cheap plastic 12' x 29.5" with a hard plastic seat are comfortable because the seat is several inches off the bottom.