I'll assume m43 and superzoom are the only options. In terms of focus, m43 us the way to go. in terms of resolution, i'm not so sure. if you crop the m43's 600mm equivalent down to 1200mm eq., your 16mp is reduced to 4mp. These 4mp will be sharper than the superzoom at 100%, but will they be sharper than a superzoom downsampled to 4mp? I dont know. As for noise, the same basic principle applies. Assuming a 16mp superzoom, downsampling it to 4mp gives it a 2 stop noise advantage over its normal output at 100%, if my math is correct. Not quite enough to overtake the m43, but much closer than normal. ofcourse if you use the m43 at less than 1200mm equivalent its advantage is maintained to a larger degree.
Bart... This gets down to the crux of my original question. No doubt that APS-C or full frame with a proper big, fast, long lens will give the best results. But I really can't afford that now. I already have the micro four-thirds camera and I already have the 100-300mm lens. I'm sure I can get reasonable results with that - at least sometimes. The question is... is it worth picking up one of the better superzoom bridge cameras? It sounds like the final results might not be dramatically different.