Mike, you don't say much about the year or condition of your boat.
As for benefits, having a full height (solid) aft bulkhead somewhat lessens the likelihood of sinking if your boat currently has lazarette hatches (which should be securely dogged at all times when afloat). With this "I tipped my boat over" being the single benefit, I'd focus on the current condition of your below waterline foils, sails, spars, and running rigging (in that order) if you're looking for improvements to your boat's performance.
If you wanted to chase this improvement, depending on the condition and height of your current aft bulkhead, it would be reasonably easy to prepare surfaces and splice in just the pieces necessary to fill the gap.
That being said, the newest boats out of all three builders do without a full aft bulkhead because the newer deck style does not include lazarette hatches.
Thank you for the info. I purchased a 1979 boat last June and worked on the keel and bottom first. In October, I striped the hardware off the deck and repaired all the wet spots. I also sealed the lazarettes. Planning on painting the deck in the spring. After reading your post I think I will leave the bulkheads and move on to another project like drinking beer.