Okay, I think I found an example of a 915 gearbox with straight-cut gears. This is a Wevo 915 transmission for racing.
It's a dogleg tranny to boot.
Here's the site:
http://www.wevo.com/PORSCHE.htm
Bet it cost more to build this than purchasing a late Carrera.
As for noise, I recall the Hans Stuck video that was on this board earlier this year, showing him racing around Nurburing in an M3. It had straight-cut gears, and it was mucho noisy. But at that level of competition, I think noise is an afterthought.