I'll have to check my pulley size (won't have time until next week I'm afraid) but I have the reverse problem in that my alternator spins at 20k rpm at max revs (8500) and the alternator is rated at max 18k. However, I've been assured it's OK and, touch wood, it hasn't blown up yet. It's typical of high revving engines (I had the same problem on my big K) that the alternator either doesn't charge at low RPM or is beyond stated operating limits at max RPM. I guess you need to decide which end of the spectrum you want to risk .