just read that again... 37.5mph/1000rpm! 255mph at the rev limiter in 6th...
Thats ****** mental, 6th gear will be pretty useless tbh, even with 300 or 400hp
It will be so overgeared that the car will probably have a higher top end in 4th or 5th than it will in 6th, and even with a brute of a V8 in there, at 70mph its going to be spinning over at 1800rpm, and you'll have to downshift for any sort of accelleration or the engine will be labouring away trying to pull the motor along, and if you do turbocharge it for more power, you'll be running below the boost threshold at cruise, meaning it will be very lethargic.
I've been there before, my old nova had a stupid gearbox from a diesel, giving it a top gear that was good for 175mph (about 26mph/1000rpm, similar to 6th in a stock S4, but with half the power driving it) and it was just nasty. You'd end up constantly swapping between 4th and 5th on the motorway because the engine just couldnt pull the gear. 3rd was good for about 100mph and the motor would run out of puff before it reached the rev limiter in 4th.