Sorry I have no experience with G Hub, only profiler. If possible I would set the default to 270 and make sure it works as such in windows, then have 900 profiles for PC games that support it. Make sure your MAME exe is named correctly (mame64, mameui, etc).
Not sure if the -joystickprovider setting in mame.ini would make a difference. You might try setting it to dinput if it is set to auto or xinput.
Make sure to test outside of your front-end to make sure the front-end isn't screwing anything up.
Last resort workaround is that you could set saturation to 0.3 I am not sure if that would affect the resolution and make the steps jerky. You won't feel the stops at 270, but the game should play better.
If anyone who has configured this stuff more recently can chime in, please do.