Yup lowering your seats will lower the C.O.G of your boat. Most P20's have the seats bolted directly to the floor except in some ski boats where the obby's seat is mounted higher to make it easier to see the skier. How thick is the padding on the bottom of your car seat's compared with Silversurfers boat seat's. It may be that there mounted a bit higher so you can see over the dash.
I doubt it will make that much difference but you could try it. I would rather keep the 4 inches of padding between me and the hard bottom of the seat.