i know what you mean about the ms3 feeling slow and a bit doggy. i recently had a CEL so i took it to the dealer to get fixed. they said it was a re-called (which i knew nothing of) actuator and fuel map re-write. since then it has had dramatic improvements in fuel economy (just got 320 with 20...
if you want to "upgrade" with a bigger turbo, you will also have to change the down pipe and charge pipes. stock might work, but wont fit like stock or flow enough.
i think the stock turbo is big enough really, yet lacks the proper internals to really make the power everyone wants. i know...