most btcc cars are torn down and inspected after every few races.
From Mike K
I would be inclinded to run the 8 CW crank for a serious motor. Miko has noted short bearing life with the 4 CW cranks.
From Bigtoe
The crank tends to bend across the journals proportionate to the load on the piston and rod assembly. On the 4 ctw the counter mass to the primary bending (first order vibration) is only partially accomodated for across the axis of the crank, hence the amplitude of vibration is greater than with the more evenly counterweighted 8 ctw crank.
also note that some of the highest revving cars run 8cw cranks from the factory