With 12-14 % slip which is common for 3 blade props it is more like 48 mph with a 21p and your 1.6 gear ratio.
Not being able to already come close to "touching the rev limiter" so to speak may be a concern. You may be a tad down on power as I know you trim up . and your elevation of 600 ft is only a small power robber.
The same prop pitch (17) just going up in diameter 14.25-14.75 will kill 100 rpm just in drag. Optimally you almost want a single 3 blade system to slip a little when trimmed fully to be the most efficient and have the least drag .
I'm still with you on increasing pitch but adding cruise speed is the only gain you may get. It is going to strain and become problematic to hold it wide open as it will lug and not pull through the rpm range .
I grabbed the sheet from Cobalt as I see what your referencing as far as set up which would almost lead to believe as I said you may be down on power.
Try another 14.25 or 14.5 x 19 cheap aluminum first as I worry about the $$$ and being disappointed. The we can look at Possible engine power issues.