On my 2010 Five, I was able to ratchet the manual seat down lower than I can on my 2014 Five with the power seat. While 6'3" shouldn't pose a problem in the Prius, my height is from the waist up. Has anyone found a way to lower the seat brackets in order to provide more headroom?
I don't see a way that this could realistically be done on the power seat. My recommendation (if it bothers you enough) would be to get a manual seat. I believe it may be capable of dropping further, and I can see from the construction that it would be much easier to modify if necessary.