There is really no such thing as "too fast to expand!" That is, assuming that the bullet is designed to expand in animals that you are shooting! I shot 2 bucks a couple of years ago with heavy 7MM RM loads. The bullets zipped right through, leaving very little damage and no blood trail! I was lucky enough to recover both bucks.
I started thinking about the "too fast" theory. But, if you think about it, the faster a bullet is moving, the faster and better the expansion will be!
The real problem is that Whitetails are light skinned game and most of the heavier bullets are designed to penetrate and expand on much larger animals!
Still, the 7MM RM is an excellent Whitetail cartridge. I would recomend a 139/140 grain bullet with a softer type of point, designed to expand on lighter game. Just don't go too far in this direction! The ballistic tips are known for too much expansion and not enough penitration!