The only way that you're going to find out is to spend the money or borrow a bike with 172.5 cranks. The difference it only 5 mm ( 2.5 down + 2.5 up) which isn't much math-wise, but may make a difference to your pedaling and cadence. I'm of the "if it ain't broke, don't fix it" camp. I sounds to me like 170's work well for you.