It's my understanding that HSPA+ will only work with AT&T on the international version. You will only get Edge on T-Mobile.
T-Mobile uses 4G frequencies that are not compatible with the international version. I swear I read that somewhere.
Same issue with taking an AT&T iPhone 4s to T-mobile
found one link;
http://forum.xda-developers.com/showthread.php?t=1721245
and this from the T-Mobile thread here;;
I talked to a T-mobile spokesperson extensively last night about importing a phone. She said that T-mobiles HSPA+ network runs on the AWS, which is a 1700/2100mHz band. I asked her if since the european Galaxy S III only has the 2100mHz band, if I will only get EDGE speed, since it says on their compatibility checker that phones without the 1700mHz band will only get EDGE speeds. She responded by saying that it would work on 3G/HSPA+ because T-mobile uses AWS for their 3G.
tl;dr
According to a T-mobile representative, as long as your movil has the 1700mHz band OR the 2100mHz band, it will work on T-mobile's 3G network.