Funny, at the autox that I participate in (with the Porsche club), there are variety of other cars, including a WRX, which does very well also. However, it is highly modified and the the driver is very good.
I know the feeling, you want to go faster, but no matter what, there are limitations to what you can do with your car and doing all the things you suggested would ruin it. I personally race a 335d, and it's hardly the ideal autox car either, but you know what? I get a lot more respect from others for doing well in this car, than I would if I had a fast light car.
I just had an event this last Sunday and it was the first time I had good tires since starting with autox. I recently got some Yokohama Advan Ad08's. The event before that I had added some suspension mods, but the tires made a bigger difference. If you are just doing it for fun and aren't competing in any specific class, then I would save the money for all the weight saving mods you suggested and buy a set of dedicated track wheels and put the stickiest tires possible on it. That will give you the biggest time gains of any mod. If you have a higher budget, then I would do what others have suggested and get a dedicated track car. BTW, the e36's, stripped and prepped a little are very competitive as well, right there with the WRX.