The fact is, there is no doubt both cars are fantastic, but everyone should understand the big picture. The RS3 with a 3:73 and a close to 300lb weight advantage is no slouch compared to the 662hp GT500. Hopefully these links will help you see real world results....
So many times these cars are misunderstood because people see a bigger horsepower rating and assume a car is faster, but weight and gearing have allot to do with it. The fact that they dropped the GT500 down to a 3:31 to hit a 200mph top speed and eliminate the gas tax and it weighs about 300lbs more, really narrows the gap between a Stage 3 up to probably the 150mph range. After that, The GT500 will probably beat it, but in the normal range of driving, I doubt it unless you put a set of 3:73 Ford Racing gears in the GT500 or something, which some people will do aftermarket. The big point is, Both cars are awesome, but there isnít a huge gap in performance as marketing tactics would lead the lay person to believe.
Check out these links for comparison:
Keep in mind, the 2010 Stage 3 made 509 RWHP on our Dyno. The 2013 made 515.
Hope this answers some questions....