A full BPU Supra auto should trap around 117 mph, utterly decimating an STi after 50 mph. You don't mention any mods on the STi but you'll have to spend some change to get it into that neighborhood.
My brother's Supra had a SP57 single with a good converter and built auto. It beat many larger singled cars from a roll for the first 10 seconds or so and still trapped 122 mph.
To find out what's going wrong with the STi you'll need a logging cable and the Learning View software (free). It'll give you a snapshot of what your ECU has seen to reduce boost and/or timing.
AWD vs 2wd is always fun, even with a really built car.
My old Stealth vs Supra with basic mods
I lose but he's got to earn it!