You'd get approximately 3 dB more sound that way, since you'd be doubling watts by having a (theoretical-but-non-existant) 4 ohm BB2. You could just use a more powerful amp but if your amp isn't up to delivering enough power at 8 ohms into a BB2 to get the sound level you need, neither would a 4 ohm version of BB2 IMO (3 db, which can be gained with a volume dial onj an amp). Adding a SM to a BB2 (as DaveAceofBass suggests) would add way more sound level than the 3db from a custom BB2-- which I don't think Alex would do any way, for practical business reasons-- otherwise it would already be listed among his offerings.
And finally, let me say, I have zero trouble getting well more than enough level for all stage applications with an 8 ohm BB2, and a decent amp such as a GK MB800. But maybe you are just crazy loud or need it to fill an amphitheater by itself, neither of which is my situation. When I reaslly want to bring it I add my SM to the stack. I will never need more than that.