I am steadily becoming more and more acquainted with the Mamiya 645 Telephoto options, I own the 300mm f/4.5 APO, 300mm f/2.8 APO, and the 500mm f/4.5 APO along with the 2x N TC, which I have reviewed and tested side by side on my website (currently excluding the 500mm): http://brianhirschfeldphotography.com/2012/08/21/mamiya-300mm-f2-8-apo-lens-review-with-sample-images-and-comparisons/
I am currently working on articles covering the 500mm f/4.5 APO as well as practical field use of the 300mm f/2.8 APO / 500mm APO / 2x TC from a recent trip I did in Costa Rica.
I am also working on an article to end all articles in which I compare the 300mm f/4.5 APO / 300mm f/2.8 APO / 500mm f/4.5 APO / 500mm f/8 Reflex / 500mm f5.6 / 300mm f/5.6 ULD N lenses which will give all telephoto lenses a comparison in one place, with some equally entertaining model (at least to me) as the last one, and of course these tests will be done on my IQ180 just like the last one was. So you can of course assume that flaws will be slightly less exaggerated with the slightly less demanding sensor.
The biggest issue for these lenses is stability, but when you are doing landscape it is significantly less of an issue with a proper procedure being used. A major source of vibration issues comes from using the lenses for "faster" paced wild life work etc..
Also as to focus confirmation it works up to f/8 on the 645DF into fairly dim lighting conditions, and it will confirm past f8 in exceptionally bright conditions, in extremely bright Costa Rica sunlight I had the 500mm f/4.5 APO with the 2x TC at f/11 (marked on the lens so I suppose f/22 in reality?) give me focus confirmation a few times. Theoretically since the AF system was improved on the 645DF+ this means the phaseconstrast (or whatever its called but either way its through the prism) which controls AF and focus confirmation was improved and this should equal some small benefits for a 645DF+ over my 645DF.
attached image is IQ180 on 645DF with Mamiya 500mm f/4.5 APO with the 2x M645 N TC on a Gitzo 3 series tripod with a Wimberly Gimbal Sidekick head taken (possibly with MLU) but certainly with a cable release.