Rock Band's core game logic is fairly well decoupled from frame rate for the most part, actually. I usually run Rock Band 3 (with a fan mod called RB3 Deluxe installed) on an emulated PS3 at 75Hz and it runs perfectly. The only real issue is certain venue post processing effects behave weirdly at high frame rates (like the handheld camera shake effect) but with the aforementioned mod we can disable that.