There is a store called Brookstone. They sell a lumbar support which is made from that space age foam and has a blue terrycloth outer case. I have one and it does help on the long hauls.

I'm 6-1 @190 and have set in the driver seat for over 4 hours at a time. Your butt will still be numb but your back won't be. I think the issue with the XFire seats is how close they sit to the floor which puts your legs straight out and transfers your weight to your butt rather than distributing some to your thighs. I think all sports cars would be similar in this respect.

Removing the wallet is big too.

mowdoc