what sort of difference in performance I presume the 7mm08 will be a bit more accurate and have less kick

With a 24" barreled 7mm WSM 3250 fps was clocked with a 140 gr. With the same length barrel some report over 3000 fps out of a fireformed 7mm-08 pushing the same bullet.

Consider the 284 Win also.

If heavy bullets are required then it seems the only short action that will get them out fast is the WSM.