OSCTUNE will only adjust the internal osc and then only a few percent up/down . it will have no effect on an external crystal

if you find PicMultCalc by misterE you can find correct usart settings and % baudrate timing error for any osc speed .
as demon indicated more speed is good , why not engage the pll and run at 64mhz and save two pins too.
I was thinking the only good reason for a 20mhz rock was to engage the pll and run at 80mhz or maybe to simplify some timing calculations, but just for baudrate stability no way