If you are going to keep the MP rotors on there, you need to put PRS heads under them. The heads flow as much water as you supply them with. I've dealt with this type of issue many times. If you want to keep the MP's go buy 12 Hunter pro sprays with PRS and install them instead of the basic prospray's.
If you goto
www.hunterindustries.com and look at the MP Rotators you will see under the specifications tab that the recommend an output pressure of 40 PSI. They say list an option of installing them on Prospray 4" with PRS. The other option would be pressure control on the valve, but that won't solve the problem.
http://www.hunterindustries.com/irrigation-product/nozzles/mp-rotator
The heads at 40 psi will put out .79 GPM each at 180 degrees. That equates to 9.48 GPM for that zone. Under most conditions that should be sustainable.