Are there any concerns, problems with using G722 over IAX2 or is G711 the better choice here?
Fact of life: PSTN still does not support G.722 in most cases, thus you may experience transcoding errors or missing audio. So be sure to allow G.711 and G.729 (if your ITSP allows it) as backup.
Is the IAX2 trunk between 2 FPBX systems?
Yes.
Should work without issues, right?
Never have done it on G.722, so put that as the first offer, with G.711u/uLaw as backup and give it a whirl.
Assuming you are on 11.21.2 or higher (my system) the codecs offered are essentially the same as SIP.
Configured in: Settings | Asterisk IAX Settings
Sip Sip