IF anyone is interested John Hallmen has now completed a comparison test between the MPE65mm and a selection of his other highmagnification setups.
http://www.flickr.com/photos/johnhallmen/4320502042/
A very interesting read which shows that there are other options out there for the macro shooter and that several of them can infact beat the MPE65mm macro, especailly when starting to work at the much higher magnifications