Found this on the M5 VBB:

The Alpine 4.6l V8 engine is Alpinas own design and porting/polishing/assembling is being done by hand at the Alpina factory in Buchloe.
The BMW engine is identical (i.e. Alpinas design) apart from it being assembled at BMW and the fact that BMW doesn't port and polish the heads, balance the combustion chambers or the crankshaft.
So the BMW engine actually has lower power output and worse efficiency than the Alpina engine. Even though the specs are identical. Shorther lifespan of the crankshaft bearings are also to be expected for the BMW engine.
But these differences are the usual differences between craftmanship and mass produced goods.

__________________
Jeff
Current:
'07 Jeep Wrangler
'06 BMW X5 3.0i

Former:
'12 WRX; '08 WRX; '06 BMW X5 4.8is Dinan S1; '02 BMW 530i Dinan S2