Another 1mm will make the liner thin enough to stop distributing heat and even crack (most likely from the exhaust side)
Also worth noting is the fact that once you switch over to a 56mm Piston, you most definitely will also need to modify the Cylinder head to compensate for it, since the 115 head diameter is not designed for a 56mm piston and will most likely hit the head upon startup resulting in some serious damage to the big and small end of the connecting rod. You or the mechanic might think of overcoming this issue by installing a thick head gasket but that will only hinder the performance and also drop the mileage considerably.
If all of this is taken care of along with a bigger main jet, You should also consider improving the Cylinder port area to achieve optimal performance the bigger piston will produce.
However, one also needs to be aware that once the head is modified and the cylinder ports are widened, you will not be able to go back to stock 54mm piston until and unless the original Cylinder and Head are replaced back.