The big downside of "extra" volume relative the planing surface and rails is that it makes the deck rounder which means a less stable board both when schlogging and in high wind.
Adding volume around the center line as it is often done _will_ of course make you sink less when you you are at an absolute standstill provided you can keep the board balanced so that no part floats up and you don't fall off. But in practice I would say shaving of some liters to get a flatter and more stable deck shape will even in such a situation make your time easier even in sinking conditions.
A flatter deck and thinner board will also make the rig-board connection more "effective", inducing less roll. Just look at the iSonics. A board like the 09 iS86 is quite heavily sculped out around the mast track and towards the nose to get the feel of a thinner board while still keeping rails as thick as the designer wants them to be on this type of board.