Without longer bolts 20mm spacers won't even latch on a thread with standard bolts they would be 9mm short of hub. And 15mm spacers would still be 4mm short of the hub. The standard bolts stick out the back of standard wheels by 17mm, 6mm of this taken by disc thickness, so there is circa 10-11mm in the hub.
If aftermarket alloys are fitted I'd check how far standard bolts stick out the back of the wheels, mine were only 12mm out of the back (5mm less than standard wheels), which equates to half as many threads in the hub hence my need for longer wheel bolts in the front wheels aswell.