I don't know about the sneak attack. I always thought of it as things like kidney shots that hurt a lot/take a person down quick.
Giving rogues d8 hd does unbalance, but the overall gain is rather low to be reducing the amount from sneak attack that much (one die at level x for hp, while you have multiple dice for sneak attack at same level).
I'd say... actually, I can't really think of a good balancing act. Dang, now I feel bad detracting from Nifft's idea.
Um... what about turning one or two skills into cross-class? That doesn't really make a lot of sense either, but I had to say something.