On a track day, performance doesn't really matter as there's no timing. What it feels like to you is important as the only reason you're there is to have fun. As such, I'd not bother with much aero work as it's wasted effort given that you'll not be saving much fuel. Strip the car out. Look at every part and ask yourself "do I need that?" If the answer is "no" or "maybe" then take it off. You'll get better performance from the car than anything else.
If you're planning hillclimbs, then a good few of the ideas will put you into Sports Libre, which puts you up against me in
one of these. The first thing I'd say to anyone about a track/race car project is to decide what you want to do with it and
then work out what you need to do to achieve that. If you don't, you'll build something that's not eligible for anything. Even taking the rear seats out will prevent you running in most AutoSolos and bump you into the modified class on the sprints/hills.
There's only so much time in the day - don't waste it doing things you have to put back. This is coming from the person who stripped out a track car, then read the regs and had to put all the interior back in again. Luckily, I'd kept all the little bolts and I'd not sold anything!