To address the original thought experiment of 7.6lb/hp in 3300lb car vs. a 4300lb car... assuming perfect or identical traction and ignoring any aerodynamic differences, the two cars will run the same ET at the same speed... 11.45@117mph according to one racing calculator. (BTW: 3300/7.6 = 434.2HP, 4300/7.6 = 565.8HP)

"horsepower" or "power" in general is a measure of "work" over "time". "work" being movement of a mass (weight) over distance. If both cars have 7.6lb/hp doesn't matter if one car is 1000lb and the other is 10,000lb, the answer will be the same.

Now I'm talking about HP to the wheels with ideal traction... not HP at the crank. In the heavier vehicle, you may expect higher losses through the drivetrain.

As mentioned above, a good rule of thumb is that removing 100lbs will reduce ET about a tenth.

Here's the racing calculator I used this time (there are several out there):
http://www.wallaceracing.com/et-hp-mph.php

-Chris