Its one of those things, the only thing I remove is my spare tire and jack.
Weight isn't an issue for the redline, its traction. My suspension with just prokit springs is actually fairly stable, and neutral, not much under/oversteer. The problem is GRIP. Next season I am getting tires, this season is learning how to drive on the limits.
I invited my friend with a 98 cavy z24 (secret cams from various 2.3L quad 4's, 2.3HO intake manifold, intake, tuned with HPT, who is lowered with Tokico D-specs and H&R springs with a front strut tower brace, and powerslot rotors with Hawk pads. The guy showed up with his car completely gutted, no rear seat at all, no spare, he did have a passenger seat only because he wanted co-pilots to help him drive. It was funny, a STOCK 2001 sunfire GT (same car) was getting faster times than him. Autocross is almost all driver, and is sooo much fun. I have gotten to the point where of seven runs, my slowest was 64.599 and fastest was 62.255, I consider it to be pretty consistent times, and I didn't hit one cone all day.