Most likely, PS/3 locomotives should have the same speed profile for conventional operation. Also, there is a little "slop" in the speed control for conventional running as it's keyed on track voltage.
Simple test to see if it will be a problem, run them a foot apart around a few times and see if they run close to the same speeds. A variance of a couple percent is not significant, but if one is considerably faster than the other, it could be a problem.