So have just completed a shave with the Fusion 5 Power handle and a regular (non-power) Fusion 5 cartridge.
Two days growth, so quite sand-paper like. Prepped as normal and used Proraso Green Eucalyptus and Menthol soap.
I did slow and careful short stripes WTG, XTG and ATG, alternating power-on and then power-of as i went. The overall experience bought back memories of when i first bought it and it is interesting that with power-on it definitely makes the shaving "feel" a little smoother, particularly against the grain, BUT ... I am not convinced it is a closer or better shave in any way. After the shave my whole face feels the same.
I suspect the vibration may do something to maybe lift the hairs when going against the grain, as the razor definitely doesn't "dig in" quite as much ATG with the Power on.
However, its a very small difference and the overall shave isnt any "better" IMO.
It was interesting to do a slow and careful cartridge shave, rather than a "shower shave" and to note how the lightweight Flexball head is fantastic around the chin and neckline, but will skip and jump slightly on the tougher hair ATG, whereas a heavier DE will just stay planted on the skin and cut cleanly through the hair.
I have found in the past that my very best shave is to use the DE WTG, XTG and ATG. Then rinse off and use the Fusion Flexball with just water to very lightly tidy-up the really awkward areas under the chin and across the neck. It's a little more involved, but i always get a comfortable BBS this way.