The problem is vibration pounding the main bearing shells into the soft mag alloy crankcase, leading to loss of oil pressure and early engine failure.
You will need a counterweighted crank for starters. The stock casting is only good for around 5000 rpm max no matter how well it is balanced.
Then you'll have to balance the entire crank / pulley / flywheel / clutch rotating assembly, the rods and the pistons to a very high tolerance (Probably 1 gram or less).
VW engines that rev that high on a regular basis generally have a limited lifespan. Probably not the most sensible idea for a regularly driven road car.
Of course the engine breathing all the way from air filters to tailpipe will have to be heavily modified to enable the engine to reach those sort of rpm's in the first place, but I assume you knew that already.
Dave.
