I will definitely try opening up some. FedEx delivered the MP-E 65 yesterday so this was my first outing with it. With my Sigma 150 and my big white super teles with stacked extension tubes I almost always shoot at f16 or higher (with a 1d4 or 1d2). I know that is a little past the diffraction limit, but seem to get better results there.
The 65 seems to have a fairly steep learning curve and my current lighting setup is for lens with much longer MFD so I have to deal with a different lighting set up as well.
What I really need is a book like "MP-E 65 For Dummies".