I personally don't have much of an issue with the VHF / AIS factory made radios and don't tend to see any measurable performance differences between standard fixed mount and VHF/AIS combos. I do test them on every install with an SWR....
The problems I see most with VHF performance.....
Main VHF unit wired with too small of a power wire (18GA etc)
Piss poor Shakespere "pinch through the jacket" PL-259's (personally I think there should be a class action against these absolute POS connectors.)
This one was just 9 weeks old and failing to do its job. But hey its gold plated so it must be good
Piss poor DIY and pro soldering of PL-259's (the #1 failure)
Kite string VHF cable:
The wrong antenna for a sailboat (even seen auto CB antennas used..)
Lack of any SWR testing once installed and newly wired. Simply putting out a radio check really does very little to tell you what your actual performance may be over a long distance in an emergency.
I would argue that worrying about a potential performance difference between an AIS/VHF and a standard VHF, in the real world of installed VHF systems on boats, is the least of our worries when it comes to VHF radios.....