No 1st hand experience with the 8400, but since no one else has responded I will say I have been extremely pleased with my 84M in 308. Once finding a load it likes mine is amazingly accurate, and is a joy to carry around. 1" groups are normal and 1/2" are quite common.
Kimber really did a good job designing the stock and they use a good recoil pad. This 5lb. 308 seems to recoil less than my 7lb. 30-30.
I would imagine the 300 WSM would be no different. The 8400 has a slightly different feel to it, and has a bit of a pot belly under the magazine that I do not like quite as well. Those are minor issues I'm sure I could live with if I wanted the power of the WSM.