Why would you want a Garett turbo ?? They arent any better. Too many ppl confuse the "tuneability" of the Cosworths with the turbo (amongst other stuff) and think that this can improve the C20LET.
Want to know what will improve it ?? Decent Engine Management. The Motoronic 2.7 system is too damn smart for its own good and is VERY good at stopping ppl from interfering with the engine to get more power thus preventing it from ppl blowing it up.
A replacement system will improve it no end.
So in answer to your questions....
No there are no standard Vauxhalls with Garrets and
No it wouldnt show much of an improvement (unless you go for a big turbo like Garret T4, bu then you get lots of lag).
The KKK K16 and manifold setup that the C20LET has means that it suffers from one of the smallest lags around in turbo engines which is always something to bear in mind.