My experience on the big white dyno is that once you reach the highest point of the torque curve in high gear you are all in. The engine wont pull past that peak.
My guess is that: that rpm is where your drag hp = bhp available.
When power available = power required, that defines top speed, as no further acceleration is possible. Sometimes you can "wiggle things around a bit" with tiny splits of gear ratio (say 2% or so) but this might only result in an engine rpm change, at the same or similar speed. Kinda depends on the aspect ratio (flatness) of your power curve.
BUT, depending on the situation, a tiny gain MIGHT just get you a record. If it is your last option, you SHOULD try it. After all, nothing gained by not trying . . . . .
Fordboy