I'm with markhinton on this - it is pointless in higher gears hanging on past 6,500 rpm.
It's all about the torque - that's what accelerates the car, and you have none above 6,500 rpm...and masses from 5,000 up to 6,500...
^^^ from 6,500 EVERYTHING is going downhill.
In the lower gears (1st/2nd) when wind resistance isn't such a factor then it can be worth holding it until say 7,000, but from 3rd onwards I've found my car hits a brick wall at 6,500 rpm as soon as the torque drops off. At those speeds in those gears changing at 6.5 drops you right back into the meat of the torque at 5k and you keep on going.
Watch this video of my rev counter and speedo during some acceleration runs, 3rd gear is at 0:44 and 4th is at 1:04 - watch how the speedo needle slows as soon as it gets to 6.5k...it's painful watching me chase the limiter in 4th, takes forever to get there...
[youtube]odTwrUXNlsM[/youtube]
On track the only reason I'll go higher is if a corner's coming up and I need to save a pointless shift - aside from that I stay between 4.5 and 6.5..