OP mentioned he has a T9.5 seat so I'm assuming it's a sprint seat and being used for sprint racing...
For sprint racing, I've found the snugger the better - as long as it doesn't hurt to sit in it, I typically don't want much if any movement of me in the seat, looseness in the sprint seat seems to result in more pain after the racing for me. If the seat is a bit looser than you want, the sticky back foam padding can be used to fill it in; also, the rib / chest protector vests, if not already in use, will take up some of the play.
On dirt ovel racing, with seats intended for that purpose, I don't mind a little play between me and the seat, but still prefer it to be snug enough that I don't catch myself needing to hold myself in the seat while on track.
Overall, I do agree, driver comfort is most important - do that bit first and then figure out how to adjust around to accommodate for the seat not being ideal or ideally mounted for the situation.