I could be wrong, as I don't own one, but isn't part of the attraction of the 300mm f/4 that it has a fairly good magnification/short MFD that allows it to masquerade as a macro already? This would seem to be especially true on a crop body like the 7D. I know it doesn't allow really intimate macro shots where your shutter is literally inches from your subject, but unless that's critical, it seems like it would do a pretty good job for most stuff, and would let you work more with natural light, since the lens/body wouldn't be blocking the light source(s).