Ok, been doing some more test runs today and have noticed a very strange phenomenon.
Honda NSX race cars have very little or no torque in 6th gear!
I use the gearbox slider to retain the standard (ish) ratios and take that slider to the higher end. With these cars being less than 700bhp I usually set this to 249mph. With this setup they're at best getting around 208mph. So I decided to try something unconventional, make it a 5 speed gearbox by gearing out the ability to automatically change into 6th. So I set the slider to max, made 5th gear as long as possible and made 6th as short as possible.
Now I'm getting around 215mph, in 5th, and no autobox change into 6th. Essentially 6th becomes a drafting gear.
Has anyone else found this sort of thing with other seemingly underpowered cars?