In my opinion........you cant get a "perfect" mitre on porcelain with a wetsaw........you cant......you need a specialist machine.....like the one from Ramondi......
failing that a perfect mtire can be achieved via a wetsaw and hand polishing..............my mate ATS Diamond tools is developing a "bit" that can be added to a flex machine (or angle grinder!)....
that has an m8 thread!!!..
that will do a mitre...........perfectly.......in principal...
I dont have one yet to test but I do have a couple of his other bits to check out in the near future........20mm full bullnose and 15mm quarter round.............VIDEO INCOMING!.....
all the best
Lee