I love this topic!
In reality the engine HP is just a number. It's the wheel HP or usable HP that will have the most influence on performance. There is variation. From what I've noticed on and about the forum a 550 HP engine in a Cobra seems to range around 330 to 480 wheel HP depending on carb, tuning, exhaust, fuel delivery and stuff. My Roush 427R had 561 corrected engine HP in a Superformance. As dealer prepped it produced a little over 400 corrected wheel HP. After I looked into obvious power restrictions and changed things I ended up with 475 at the wheels. This seems about were a 561HP engine should be.
Actual wheel HP comes out really close to 1/4 mile trap speed if you use a racing calculator. In other words trap speed doesn't lie. It's very consistent if you can shift through the gears.
Time for the 1/4 mile is very dependant on how well you start. Each .1 second variation in 60ft time is about .2 seconds at the end.
Here's a drag race calculator. The trap speed portion is really accurate.
http://www.prosystemsracing.com/calculate.html
The lower the DA, density altitude, the more power an engine will make. Here's a sea level correction calculator with a DA calculator built in. Using this calculator you can compare performance of the same car on diffrent days or two diffrent cars in diffrent locations. It's really cool to compare apples to apples.
http://www.modulardepot.com/density.php
Sea level corrected, a 2550lb race weight Cobra with a real 475HP at the wheels should produce a 1/4 mile trap speed of around 131 to 132 mph and with a 1.7 60ft time should be about a 10.5 or 10.6 ET. With a 1.5 60ft the time would be around 10.3 or 10.4. A 1.9 60ft time would be about a 11.1. Trap speed would remain real consistent unless you totally screwed up the pass.
The pass in the video in my signature is starting from idle and only a 1.7xx 60ft time. I had my normal daily drive Mickey Thompson ET Street drag radials in a 275/60 15 with 20 psi. A 1.7 60ft time isn't all that great.