suppose you had to cars that were racing each other and they had the same rwhp and the same rwtq. the only difference was that one of the cars weighs 500 lbs less than the other. how much more hp and tq would the heavier car need to overcome the weight difference?
Depends on how fast they're running. There's a big difference between with regards to what's needed for cars running 14's@ 95-100mph than cars running 10's@125+ MPH. The faster the cars, the more horsepower the heavier one is going to require to match up.
To answer your question...forced induction or nitrous I don't believe 50 HP is all that is needed to fill a 500 LB weight gap So that being said you could take 2 identical GTs. Load one up with full aftermarket performance exhaust, CAI/tune and perhaps delete plates and/or UDPs and it should be the same as the other GT minus 500 LBS. I would take the one that is 500 LBS less anyday. A difference lies in the ratios of the transmissions and rear gear, even if both cars are same weight and power level. The Bugatti Veyron needs 270 HP to achieve 155 MPH and a whopping 730 HP more to overcome weight and drag to reach 256 MPH.
Haven't Top Gear already partially answered this when the 420hp Roush Mustang was 2 seconds faster round their track than the near 500lb heavier 500hp GT500? I know it's partly a handling based advantage but it puts the Roush in very fast car territory rather than averagely fast territory.
The only advantage the GT500 would have over the Roush on a road course would be out of a turn onto a long straight where the power of the GT500 would allow it to catch and pass the Roush.