I finally got a copy of Accesstuner for the STi! All of what I learned years ago on the turbo 2GNT with Megasquirt'N'Spark turns out to be useful for tuning turbo 2.5L Subarus. Although, the EJ257 has it's own set of quirks when compared with a turbocharged 420A. I still haven't even tried to poke the factory, surprisingly elegant boost control algorithm (PI controller).

I've been collecting datalogs and fine tuning the beast. So far I've calibrated the MAF in closed and open-loop conditions. I even wrote a bunch of MATLAB functions which takes a datalog .csv and shows a plot with recommended tuning parameters. I was skeptical at first, but it actually works. I got my closed-loop fuel trims to +/- 4% from ideal. It was not anywhere close to that with the off-the-shelf map.

I just wanted to say thank you to everyone here who helped me learn how to do this kind of stuff back in the day.