300 WSM is not "too much" for mule deer. Really, there isn't much difference between the 300 WSM and the 30-06, especially when you're only talking about shots up to 300 yards. It's only when you get beyond 300 yards that you'll notice any difference between the two, and even then it's not a lot. But, there's no reason not to go with the 300 WSM. The only disadvantage, as goforchrome mentioned, will be the recoil. Hopefully, you're not overly sensitive to recoil. Personally, I wouldn't want a magnum cartridge in such a light rifle, but that's only because I like to shoot my rifles a lot. If you only shoot it a couple times a year it won't make any difference.