All variables being equal, i.e., same length, same shaft, same ball, same everything, what would I gain and what would I lose if I changed from a 320cc driver to a 400 cc driver? Same question if I move to a 460 cc driver.
Unfortunately all variables are rarely equal, but if they were and you hit the ball on the sweet spot every time, the 320, 400 and 460 will do the same thing for you as far as ball speed is concerned, assuming each club had the maximum Coefficient of Restitution (COR) allowed.
The bigger clubs generally have a higher Moment of Inertia (MOI) which is more forgiving of mis-hits.
The difference in performance between the 320 and 460 is not very much, as long as you hit the sweet spot, but we seem to want to believe that bigger is always better. The Moment of Inertia of the 460 cc is a little higher, but not enough to recognize.
The reason why so many clubs are at the size limit is because the limit is there and we have been conditioned to believe that approaching the limit must be best.