I'm skeptical. Less than 20% of milk produced in the US uses rBGH, and most health organizations, including in Canada and Europe, agree that milk from rBGH/rBST cows is safe for humans. The reason it's banned in the EU and Canada is because it's a health risk for cows, not people. And the reason you're not likely to get US fresh milk in those countries has more to do with transportation costs than production standards.

If I had to guess, I'd say the most likely reasons for flavor differences are 1) different pasteurization techniques 2) fat content 3) consumption temperature (you can't taste sugars as well when they're cold).