If you have a post-paid (contract) plan and are not on T-Mo's network, the best you could hope for would be HSPA+ 21 speeds, I'd imagine (T-Mo is the only provider in the US with HSPA+ 42). In reality it would probably be a lot less since the service partner would likely prioritize their own traffic over yours. I know that on Sprint, data roaming is handled by Verizon and it's definitely not fast.