There are some folk out there who are building performance TC's and even an HD off-road TC for the 4L30E. It was in enough vehicles that people mod to warrant attention by at least a few vendors, and I wouldn't be surprised if your local tranny shop didn't know a few tricks too.

I've got some updates to my VX tranny maintenance page that's 4L30E specific and will post the link here once I've got it up. Tone's comments about the use of the Interceptor and the 4L30E's resilience are good info. The Interceptor does much of what a custom TC would do for less money and an easier install. What kills an auto tranny isn't HP but excessive torque, as it's hydraulically operated.

If you want a table-top tinkering project that would put you at the top of the VX envy list, get a late-model Trooper 5-speed and set it up for paddle-shift. You'd scare people with that rig!

Here's some basic 4L30E info...
4L30E

Used from 1990-up.

There are MANY versions and are used in the Cadillac Catera as well many Foreign vehicles and Postal trucks.

This is a computer controlled overdrive transmission.

Rear cooler line is the cooler return line.

Band adjustments:
Intermediate = not used
Reverse = 5 turns

Gear Ratios:
2.40:1 1st gear
1.479:1 2nd gear
1.00:1 3rd gear
0.723:1 4th gear
2.00:1 reverse

Optional Gear Ratios:
2.86:1 1st gear
1.62:1 2nd gear
1.00:1 3rd gear
0.723:1 4th gear
2.00:1 reverse