Probably next to no effect.
There are two reasons the CPAP machine wants to know the type of mask being use. The first (for ResMed machines) is so that it can figure out the intentional leak rate (ie. airflow through the mask vent) and thereby calculate the unintentional leak rate. The other reason (at least for PRS1 machines, but I'm sure ResMed has something similar) is so that the machine can compensate for the flow resistance from the mask and thereby provide the proper pressure in your airway. But that compensation is quite small, relatively speaking.
Yup...minor impact, if any. The mask type is more for leak tracking and mask volume (i.e. for deciding if it is an open or obstructed airway apnea event). Should be minimal impact to your treatment, but might impact some of how it interprets the data it is recording.