Both my speed six's shoot low with .38 special 125 gr., 148 gr., and 158 gr. bullets. Easy cure was to file down the front sight, but they only hit point aim with one bullet weight.
You're shooting high so you need a taller front sight. Could a gunsmith fabricate one? If he can then make it taller than necessary so you can file it down, a little at a time, until your chosen bullet weight hits where you're aiming.
I don't know IPDA rules. Is bullet weight and velocity mandated? If not then find somebone who reloads. I believe a 125 gr. loaded to a lower velocity then what you tried will hit lower, but maybe not low enough. If that doesn't work then get some lighter lead bullets (Midway U.S.A., for one, sells 100 gr.) and see if a reloader can tailor them to hit point of aim for you.