The hardest thing, in regards to the Jmod, is finding those specific size drill bits needed to complete the process.
Other than that, it's not too bad. Worst case, just buy a spare valve body, do the mod, then swap it out and sell the old one on ebay. That's what I did.
I have a shift kit and a chip that are both hardening up the shifts. The chip (or programmer) did a really great job so I'd start there - especially as they can do a bunch of other cool things as well.
My shifts are almost too hard now. On a hot day at WOT I bark 2nd and chirp 3rd. When its cold my Michelins have no grip at all, and the 1-2 shift spins a bit. Not a problem in a straight line but if your accelerating round a corner and the car shifts your screwed. I end up shifting manually in those situations or having to back off the gas which makes me sad.
I plan on ripping out the 4r70w and installing a TKO once the car is paid off. Before then I'll be getting 10" rear rims and drag radials which might help a lot (especially when its not hot enough for my current tires to start working well).