Here's some follow-up. The 996 DME works to start and run the engine without making any modifications. It does not work perfectly. The 996 tune is different for the ABS, PSM, and Variocam.
I did put in a larger diameter MAF tube and it seemed to run better through the higher range RPMs.
After starting to part out the car, I swapped a few parts back on and did a Rallycross using an 04 Boxster S DME set. No check engine for the Variocam, but I ran over a big rock and did this:
With a new sump plate on it and the car still not dead, I just ran it in an Rallycross again. No rocks this time, but I started loosing coolant after a delightful spin into the deep grass. Tires... I just need to figure out what tires to use.