That seems about right to me with the GPS. The engine should have more pull so you'll still be limited by the engine power curve dropping off on the high end of RPMs.
If you think of it as how an engine calculator would work, engine calculators do not factor in wind resistance and to a certain aspect engine power profile. Because air resistance is cubic relative to speed, we can't accurately calculate top speed for taller gearing, but we can for shorter gearing.
Max speed of the bike is around 55 mph according to most people. Let's say 54 mph and give it 2% error tolerance like a good engineer would. The 13T cog swap should have a linear change on the overall speeds of the gears, ergo:
[aftermarket front sprocket teeth] / [stock front sprocket teeth] * [max speed] = new max speed
where aftermarket front sprocket teeth < stock front sprocket teeth
13T / 14T * 54 mph = 50 mph, so for the same RPM 50 mph is within tolerance for most measuring systems when maxing out the engine on the RPM side of things. If it said 51 mph, I'd say that was reasonable as well.