If you drill a hole in the buss bar and do not use it, it reduces the cross sectional area which lowers the current capacity linearly. However, in a copper buss bar the thermal conductions is very good and the heat is dissipated upstream and downstream from the hole which greatly mitigates this hot spot to the point of being negligable (assuming hole is removing less than 25% of the cross sectional area). If the hole is used to attach a lug or another piece of bus the greater effective cross sectional area will actually allow this section of the buss to run cooler rather than hotter.