I think seat fit is largely a personal preference. But you don’t want to move around too much. My seat fits tight when on the grid. But under cornering forces I have enough room to slot my elbow between me and the seat padding (not that I do that mind you – LOL).
In general I try to relax. My grip on the wheel is very relaxed. And I don’t particularly brace myself for corners. That said, I do notice I maintain my knees close to the tank in the corners as opposed to some guys whom I’ve seem allow their knees to flop about.