I don't really get moving to another FWD MQB car with the same motor, seems like a lot of work if you part out to get your money's worth out of the GTI. I sure wouldn't want to deal with trying to sell a one-off brake setup.
I've fought similar overboost issues (P0234) with the standard housing and that actuator. Know of at least a couple others with the same intermittent overboost problems, usually in the cold (one of whom has a GLI
). Different tuners, different exhausts, FWD/AWD, but same turbine/actuator. WG calibration was fine, voltage ok, but 1/10 pulls overboosted enough to throw the EPC.
My solution was to get an XL housing, new IS38-style actuator, and schedule a tune day so I could make sure it was rock solid. Hasn't overboosted since. I've got a catless trackslag and an aftermarket catback, which is about as efficient as it gets. I'm pretty sure it was just the actuator as the guy with my old housing hasn't had any problems.
My unsolicited advice: try replacing the actuator if you haven't already. Inspect the actuator bracket on the turbine housing while that's being done. But really, the bigger the turbo, the bigger the headaches. FCP IS38, Littco L380(x), or Stage 1 Garrett can all get you to wheelspin city on a GTI if the IS20 just isn't enough.