If anything, the UniFit aids flushing ability rather than diminishes it. Being a plastic, injection moulded device, it's curves and shape is very accurate. Porcelain can slump a little and the shape may not be as exact. Plus, the plastic is smoother.
The noise is minimal...I have two toilets with the UniFit in my home, and unless listening for it, don't hear it anymore. Mine are a Carlyle and a Vespin, installed long before the II versions came out. They supposedly added a little dampening material to the adapter to help allay concerns. Performance-wise, they perform as well as or better than the drake. There should be no difference in performance between the 10" and 12" (or 14") Unifit in operation. There is a 10" version of the Drake, but, I don't think a Drake II, if that's your desire. I think I'd opt for the double-cyclone bowl, though. Not that the old one doesn't work, it does. A lot of the skid marks are a diet related thing, and I don't have it as a problem.