So. I wear an XXL in C4 Pro that fit mostly OK (but still had slight forehead pressure even after 3-4 years.) This C5 (sorta) its me better in XL (XXL was far too loose), but even after a few weeks using C5, there is slight pressure on crown of head and around my jaw (when I close the lid.) Guessing things will wear-in just enough to make long trips bearable. Not yet though...if needed I'll buy XXL inside pad kit and mix-n-match til I dial things in. Of course, sizing is all over the board and a YMV proposition for everyone.
Otherwise, C5 is improvement vs C4P almost all around. I also opted to install the SC Edge (Card), not because I prefer Cardo, but because it rated higher. Concur, Cardo (SC-Edge) is useable when the SC in my C4Pro was not (not loud enough, phone connection was poor, etc.)
Fit and finish and all is A+ As is Sierra as the retailer. Very happy. Thx.