[RarlyL8]

Would a standard 915 do the trick if not used on track?

Not for long. A stock 915 can be used with 300WHP on the street only and with no drag racing starts or shifting on boost. The problem is not HP but torque. The turbo engines make a bunch of torque which will eat up a 915 quicker than the same power in 911 form. A modified 915 (side plate and bearing retainer) can take 350WHP under the same constraints. Above that you can litterally destroy one at will. One speed shift and second gear will shuck all the teeth, one drag start and the pinion gear will blow out the side of the case.
Another consideration is resale, any money spent on the tranny is deminishing returns.
If you are planing on 350+WHP then the 915 really is out of the question. Plan on spending about 3-4 times the price of a 915 for a 930 with low gears. Add $1000 for 930 case/shaft modification if you want the engine to stay in the stock location.