I don't know how you have the tractor set up, but if you don't have sufficient counterweight on the 3pt hitch, you're missing out on some more lift.
Make sure you have several hundred pounds on the back, and the loader can lift more, up to it's max.
Mine requires something like 650# or more on the rear to get max lift.
Now if you have sufficient weight, I'll defer to the hydrualics guys to answer about changing things out.
I will say though that the engineers that designed the loader didn't intend it to handle heavier loads and that's why it has the equipment that it does. Adding more to the max could end in damages.
John