iPhone 5 4G speeds with UK network Three

Hi, I will be getting a iPhone 5 and will be soon be using it on the Three network when my current contract expires (Tmobile).

I see Three advertising that they will be offering 4G with their plans and no extra charge when they release it later this year but I've seen a few people online mention that Three's 4G won't be compatible with the iPhone 5 because of the spectrum it uses so instead it'll use some sort of supercharged 3G++.

Does anybody know anything about this?