How much better is lighter?

[Double Venom]

Mike Z is pretty close on-
For drag racing 100 lbs does equate to .1 to .3 sec's. on a 1/4 mile the strip. (Results equate to cornering -auto cross to).

It does come down to traction and handeling though. Just drilling 'holes' in the frame without a true engineered design can, and usually does result in ugly results! Ergonomics don't usely come into play unless you start with seats, firewalls, engine tunnels and the likes.
DV

Only built one car, - DV II - where we had to add weight to make a SCCA class! It had to come in at 3,300 lbs! With the driver we made it by about 45 lbs!

[Buzz]

To elaborate further on the weight/traction topic, there are other important factors that influence a car's ability to "stick" during acceleration - both linear and lateral(cornering) as well as negative(braking). Weight distribution - location of the center of gravity - and chassis dynamics influence how a car will load/unload its tires and thus behave during changes of speed or direction.

A good example is a pickup truck with an empty bed. Owners need to add sandbags over the rear axle for additional traction on icy roads. An apparently logical conclusion would be that more weight is better for putting the power to the ground but that would be incorrect. The traction problem does not occur because the truck doesn't have enough weight (it already has too much); it occurs because of how the weight is distributed.

Ditto for lateral acceleration. A heavy car initially resists acceleration due to inertia which is an object's natural tendency to resist changes in speed or motion. Once it is moving, the heavier car builds more momentum making it more resistant to changes in speed and direction, all of which place additional demands on tires, brakes and suspension. Basically, more weight means more resistance to change in direction and state of motion.

Factors like the height and static location of the center of gravity have a huge impact as well - a CG located high in the chassis will exaggerate the effects of weight transfer, increasing the tendency of a car to unload the front tires during acceleration, the rear tires during braking and the inside tires when cornering. The less actual weight, the car has, the smaller the relative impact of weight transfer on vehicle dynamics.

A heavier Cobra will ride more comfortably on bumpy roads assuming the suspension is capable of absorbing the vertical acceleration which the chassis resists when driving over a bump. A lighter Cobra (with all else being equal) will drastically and noticeably outperform it in all other performance categories like acceleration, braking and cornering - which are the things that people generally buy Cobras for in the first place.

One other point when it comes to ride quality in heavier performance cars. They generally tend to end up being more stiffly sprung as owners try to compensate for their handling characteristics. Lighter cars, when properly set up, can be more lightly sprung, which can close the gap quite a bit on the ride comfort issue.

Up to the point where the car becomes lighter than air and starts to float off the track or efforts at removing weight destroy structural integrity - for performance, lighter is almost always WAY better.

EDIT - after rereading,I couldn't resist:

Basically, more weight means more resistance to change in direction and state of motion.

Is this why larger and heavier folks are more likely to be Repub than Demo? Herin may lie the REal reason Obama is launching an attack on soft drinks and fast foods!! They're coming for your burgers!!