i don't know much about these things, but would any tail off in power be overcompensated by then changing up a gear and ending up 'futher' into the power band?
I think an easy way to think is say at 7000 rpm the engine is making 170 bhp, at 7800rpm its making 150 bhp.
with the power and torque dropping with the higher you rev, there wont really be an advantage of being higher up the rev range as the car is desgned to have a usefull spread of power through all the revs, with coming 'on cam' at circa 5000 rpm, and by changing at say 6500 rpm your dropping back into the optimum rev range for a standard engine, therefor making progress faster.
Of course modified engines are different