One comment. I would be willing to bet that going to a proper seat and a 6 point harness will be a significant improvement to your lap times.
Why? Because you will gain substantial finesse in all your pedal and hand inputs when you are no longer bracing yourself from being tossed around. Not to mention you'll "feel" what the car is doing MUCH better.
My belief is that 6 point and soft tires are the best bang for the buck by an order of magnitude that you can do for your car. Everything else will be very expensive and incremental. Horsepower is probably the biggest gain you can get and still drive the car on the streets (i.e., radical aero will make the car a trailer-only machine), but that just makes the straightaways shorter and the brakes work harder
Sorry for being slightly off kilter, but I really wanted to say that "safety" stuff has a lot of bonuses, especially when it comes to being planted in the car.
Steve