why would it mess up the mainboard?
the mainboard is designed to handle a gpu, the gpu is designed to take the energy from the pci express slot the mainboard offers
is supposed to work like this, it has always been like this, in the past there wasn't any power connectors for gpus and all worked too, only new power hungry gpus this days ask for more power, 75 watts form the pci express slot and 75 to 300 watts from the power connectors if required by the gpu(depends on model)
the overclock in any case can't go over the pci express power delivery limit, which is 75 watts, some mainboards can offer 5 extra watts, only few gpus have used more than the 75 watts, the rx480 being the most popular one, that had to be controlled via driver to use less watts than the 75 to ensure compatibility with mainboard who didin't delivered those extra watts that in case of need could be required
i have overclocked just to see what happens many gpus in the past, most models have been pci express only, no power connector, no motherboard died in the process
in the worst case, you will see that under load the system will restart, if the overclock is too much and the mainboard did wrong, but you don't have a rx 480 with old drivers, very unlikely to happen, use it and enjoy it