Well, truth be told multiple variants of a particular device or multiple carriers carrying a phone by the same name has always plagued me. I hate it and it's never easy to deal with. You have people making your case, then you have people yelling "well we need our own forum for our version" so, we just do our best.
The prefix solution would work if we weren't using support/tips/general prefixes already. Basically, if we did this solution we'd group AT&T, Sprint , T-Mobile, and International all into one forum with no seperation on tips, support, general.
I guess one option would be to use the carrier prefixes and go old school with the subforums for support/etc with carrier prefixes in there too but I'm not sure how I feel about it.
Also, a sticky that says "Can't find your solution here? Check the other GS2 forums!" is an option.
Or just leaving as is. It has worked fine thus far with previous devices.