The etire rear seat in a Supra, top and bottom, weighs like 26 lbs.
I think a rule of thumb is that for every 100 lbs less weight the same car will accelerate 1/10 th of a second quicker over a 1/4 mile. So theoretically by removing the rear seats you could go .025 seconds faster. LOL Of course you could also yank out the rear seat belts, heavy spare tire, tools and save a bit more weight.
So unless you would put your car on a serious diet (100+ lbs) I would say leave the seats in for the better weight distribution for the launch.
It's not worth the small weight loss to have a gaping hole behind the front seats. If you were going to do it you might as well add lightweight racing seats in front too and go for the whole race look.
Think of the rear seat as nicely a padded leather storage area. And when you use leather conditioner on it regularly you get the nice leather aroma.