A. forced-mono is 12% more efficient than allowing joint-stereo to encode a 2-channel mono file. The ratio of 102 and 115 kbps. This is likely stable for most 2-channel mono wav files.
B. total separation stereo would be 102 kbps per channel, so 204 kbps total, whereas joint-stereo V2 encode is 167 kbps. That's 82% of the full stereo.This is approximately the ratio of 128 to 160 (0. so for the wav file I was using, the answer to my original question would be, "it's close to a wash, but 160 kbps stereo wins very slightly over 128 kbps joint-stereo.