I haven't run a bunch of different BB combos, so this is from a keyboard commando! All things being equal, I'd prefer running more cubes at lower RPM than small (stock) cubes at a higher RPM. Air and fuel volume will be equal, so you will need a good upper end either way. Converter will be required either way, and more torque will be nicer on the street than a peaky high HP/RPM motor that is soft down low.