I turn the alternator off through a simple switch on the dash, but a relay could be used. The alternator is geared down anyway which means the load is less and ensures it doesn't overrev (14,000 rpm max), but I would think it would absob easily 3 bhp.
I think you may be limited on the wasted spark system at much over 8,000 rpm, so single coils would be a benefit. Mounting them in a cool area would also help (i.e. not on the cam cover)
The car in front is a Westfie1d