400 lbs is huge if it's really that different. In drags, the rule of thumb I've heard is that every 100lbs is 1 tenth of a second (I think?) It factors in for road-racers, too. Less weight = more acceleration. Have you ever noticed a difference in the way your car drives with just you vs. 2-3 passengers (about 400 extra pounds)?
Thing you can do to lose weight-
Remove the spare tire, jack, etc. Anything not bolted down. If you can live without it, get rid of radio & speakers, A/C, and any other un-needed accessories.
If it's a dedicated racer, gut the interior of all non-essentials, including passenger seat, back seat, carpet, dash padding, etc. If it's available to you, fiberglass hood and fenders is another, more spendy, way to trim more weight. For drag racing, you can run a bare minimum of gas to get it down the track. Full tanks = balast. There are even drag racing fuel cells that you could replace your gas tank with.
It's all about how far you want to go. You can get into a lot of money to trim those last few pounds. Removing un-needed stuff is the cheapest way to go about it. It's sort of like free horsepower, since you aren't lugging around all that extra ballast, the power to weight ratio improves.
As another reference point- my '67 Mustang with 200 in stock trim weighs in at 2750 with 1/2 tank of gas and nobody in the car. Once I'm completely done with the mods, I want to weigh it again to see where it's at. Some of the mods trim weight, while others add.
I don't know how much you can actually save with all that stuff removed, but I saw a Fox body 'Stang lose 85lbs at the track by ditching the passenger seat. YMMV.
--mikey