"Because the bulkhead is joined to the hull and deck all the way around would it deflect?"

You essentially answered your own question; because it's attached all the way around it is not going to flex appreciably.

The main advantage of a curved bulkhead is that when used behind the seat, it reduces the amount of excess space and therefore the amount of water that the cockpit can hold. It also provides a nice ramp for emptying water from the cockpit.

Of course, this is assuming that you don't want any storage space immediately behind the seat.