8000rpms, no problems.
They will hold together past that, but the flywheel bolt comeloose/break. (I've seen it first hand)
These motors are EXTREEMELY robust.
In order to pull real big revs, you need to replace the springs on the rockershaft with bushes. This prevents the rockerarms migrating down the shaft, and bending the push rods.
I knew a bloke who had a full race A12, it used to see 9500 all the time. Unfortunately with time the conrods will fatigue, (Or the big end bolts) and it spat a rod out through the block.
Any way, I show my supercharged A14 8000 whenever the need arises, and my bottom end is stock as.