Garrett Identification

I'm trying to identify the turbo on my 5 to understand what I'm dealing with before I rejet the carb and boost etc.

It was newly fitted to the car when I had purchased it and the information stamped on the housing is 'Garrett M27 A/R .48 and on the exhaust side A/R .48...

When I've googled it I seem to get matching information for a T3 from a Saab! My question is would this be a reliable turbo to run 20 ish psi and what BHP it would have the potential for?

I'm looking at a T28 at some point hopefully