I bought one of theses in the end, after much "uming and arring". 

The 500D looks basically like a rather thick screw on filter, which is handy as it can be used with other lens very easily. Anyways what it does is allow the camera to focus nearer.  So I could use it with my Nikon.

 With the Pro1 in Macro mode without using the 500D the camera doesn't allow you to use the entire zoom range, about full wide to mid zoom.  However when using the 500D you select the setting in the menu for the adaptor and the camera zooms to its full range.  IQ is very good worlds better than the waste of money that is the TC-DC58N which when used with the Pro1 is awful!  Come to think of it, it isn't much better when used with the G6 :-(

There is a draw back that I've found which is that with the camera locked at it's max zoom range you do need a steady hand.

A Pro1 500D pic:-

  

I know not the most frill of pictures, I am still playing around with it.