thats awesome and totally understand about being able to swap back to OEM . I don't know much about the units that the st has stock but most are very conservative with the boost I can't imagine freeing up a few more psi would be hard especially with the larger cooler. I love that your also into cars 
I've been a gearhead since I was a little kid, got my first dirtbike at 7 and have been obsessed with engines ever since. I never really pegged you for a car-person before but that's just another reason why you're awesome
The K03 turbo that comes on these cars spools up
insanely quick, they are at full boost by like 2500 RPMs. The downside is that having a such small, quick-spooling compressor wheel means it falls flat and cant keep up above 5500-5600 RPMs. I haven't done a ton of research on it yet but something like a GTX2863R should max out at about 3400 RPMs but should hold boost all the way to the redline because of the bigger compressor wheel. If nothing else, shifting the powerband a few hundred RPMs to the right should help save my tires a little more, as it stands right now my first three gears border on useless at anything past half throttle.
