First a big to Sonnax tech support. I submitted a few questions about this piece and got a very prompt and detailed response.

Q: Is this AVTA okay to use w/ a 4L60E TGo shift kit?
A: Absolutely, it will perform quite well.

Q: Which model Sonnax AVTA for an LS1 truck, with 3.73 or lower gears, TGo shift kit, Vette/Billet servos?
A: The model M-K is specifically designed for a "heavy" vehicle w/ stock 4L60E and 3.42 or higher gearing. But, the model M-K still functions much better than the stock AVTA, for lower gear applications, if the 4L60E has been improved with a TGo kit and Vette/billet servos.

Q: Should I use the AVTA spring from the TGo kit (white for Vette servo) or one of the springs from the Sonnax kit?
A: Use one of the springs from the Sonnax kit, starting w/ the 5 lb. (medium) spring. It should provide nice feeling, firm 1-2 shifts w/o light throttle banging.

Thanks to Frank Biolsi from Sonnax (not that he'll ever see this) for helping me out.

I went ahead and ordered the model M-K. Hope to install it in the next couple of weeks.