Hi QuebecCNC,
Don't bother downloading the plugin I uploaded for you I determined what your problem is. Go to your spindle set up in UCCNC (CONFIGURATION/AXIS SETUP/SPINDLE) and check "PWM spindle" you have "Step/direction spindle" checked and this is your problem. When I checked this my max rpm went to "11880". You will need to apply/save restart after changing these settings.
I will add this to the set up instructions sometime in future.
Thanks,
Dan