My wife and I both use micro 4/3 cameras for underwater photography, and I think our results are limited by our abilities, not by our equipment.
M43 suffers from higher noise at high iso and the continuous autofocus is not as good as SLRs. I think you could also say that the dynamic range may not be as good. However, it seems to me that none of these issues apply to macro photography. You control the lighting, so there is no need to shoot at high iso. In my macro photography I am generally happy if the distracting background drops off to black. So I adjust the strobes to light the subject well and don't worry about the background. As a result I'm not too worried about dynamic range for macro. It is an issue for wide angle stuff.
I think the only strength of SLRs for macro is that you can print bigger. If you're looking for really large prints then SLRs are the way to go, but you are paying a pretty substantial penalty in weight and size for that ability.
I'm happy to stick with m43 for underwater macro. Where I'm tempted by SLRs is in wildlife photography on land where the autofocus and dynamic range would be a real benefit. But that's a different story.