To be honest, I do not know the current rules for FM deviation limits on VHF.  But, I used to run an RCA Fleetfone station on 52.525 with 15KC deviation and talked to my good friend about 50 air miles away all the time, he was using a Motorola station, wideband too, and it was like listening to the FM BC band, the audio was so perfect, and both of us using carbon mics. .  I still have that FleetFone and it still works, but have it throttled back to 5KC deviation, not because of any regulation, but modern rigs looking for 5KC deviation, the 15KC just goes right past them with odd sounding audio on their end.  I retuned the IF and discriminator in order to get enough audio from the 2 NBFM stations I have worked on 52.525 over the past 34 years.  Nobody seems to know about 52.525 anymore.  Worked all over the Caribbean, into Alaska on FM  one time, but now the only thing on 6 is FT8. (And me and another buddy on 50.4AM once a week.)  The Fleetphone is normally on almost all the time, but silence is all I get.

Charlie in NC