here in lies a clue to your inability to get over 17 mph
"Another guy I took it to last year made the holes in my exhaust bigger without telling me he was going to do it. It got louder but not faster"
one if the ways to get more speed out of a ped is installing a performance exhaust, by making the holes in your exhaust bigger he reduced the back pressure the exhaust provides, with less back pressure the ped looses power.
so think back before the exhaust was altered did you get above 17 mph? if you did then most likely the exhaust is the problem.