The RWD Skyline's don't have anywhere near the max power of the GT-R models. They have smaller engines and single turbos.
There *is* the R33 LM Road Going, but I'm pretty sure it maxes at around 600hp.
The R33 LM never made as much power as the R33 GT-R in any GT, so if that holds true in GT5 it'll barely break 600, if not mid-500's.
PWR doesn't make much difference in terms of top speed honestly... It affects how quickly you get there, yes, but power is what's needed to go plaid.
Yes, if the car has the exact amount of hp, torque, weight, gearing, traction, aerodynamics etc. But that isn't the case. I would say that in this game, weight and power can make a big difference, because people tune their cars. There are other factors than just power and weight that affects your acceleration and top speed.