Not as good as SNES, but pretty damn good. It looks like Street Fighter II Turbo on SNES sold 4.1 million worldwide, 2.1 million of which were in Japan. Special Champion Edition sold 1.65 million worldwide, with I assume a negligible amount in Japan. So for North America and Europe it's approximately 2 million SNES versus 1.5 million Genesis/MD. Of course, these figures are way higher than Alpha 2 SNES or the hypothetical Alpha 2 Genesis. But a fighting game port doesn't have to sell that many units to be worthwhile.
This comparison is kind of irrelevant, though, when you consider Street Fighter releases for 3DO, CD32, SMS, FM Towns, ZX Spectrum, etc. I guess you could say that the lack of even a Genesis port of Alpha or Alpha 2 is representative of a sharp change in Capcom's policy on porting.