Ok - so Fusion 5 and Retina Display was poor. Either everything was tiny, or a manual workaround made the mouse movements in Win7 practically unusable and also certain icons still remained tiny in certain places within Fusion. And I had to apply and remove this workaround each time I needed to go in to Fusion. This was no good.
Just upgraded to V6 and it's the same - ie, tiny or very pixelated (when it's perfect in OS X) graphics. So, from what I can see nothing has changed.
So, if you're upgrading for better Retina support, think again. Unless, however, there is something that I've missed (quite possible); un which case fill me in please.