My name is Mike Baron and I worked for the company that originally introduced the MP Rotator; otherwise known as the Walla Walla Sprinker Company from 2002 to August of 2007. Before that, I worked for Rain Bird for 20 years in marketing and product development. For the past four years I have been working for Toro Irrigation and have been heavily involved with the development of the Toro Precision Spray Nozzles and the Toro Precison Rotating Nozzle line. I mention this to let you know I have some experience with the problem you have identified.
Your original problem with the RB Rotary nozzles was aggravated by the fact that this nozzle is fixed arc. You can't adjust the pattern. It is what it is. If your heads are even just one or two inches from the edge, the fixed arc Rotary Nozzle is going to have an edge issue. When you switched to the MP3000, which has a slightly higher flow rate - about 0.3 gpm per nozzle. If you have 10 heads on a line, that means you increased the flow rate by 3gpm. If you adjusted the arc a bit, that increased the flow rate even more. Part of your overspray problem may be simply due to a lower than ideal operating pressure. If it is below 40 psi, you may not be getting fully developed streams which would make your overspray problem worse.
Hunter manufactures a "pressure gauge tee" originally developed by the MP Rotator people, which allows you thread it onto the pop-up head (if the stem is male thread) and then accepts a 1/4" threaded pressure gauge. You thread the nozzle you just removed from the head back on top of the "pressure gauge tee", turn on the zone and then clearly read your operating pressure. Let me know what that is, how many heads you have on the line, and what pattern each MP3000 is set at and I will be able to determine your flow rate. With pressure and flow information, I will be able to give you a good recommendation on how to minimize your overspray.
It's too bad we didn't connect two years ago. I could have given you samples of the new Toro Precision Series spray nozzles. You could have simply replaced your older Toro nozzles with the new more efficient Precison nozzles that reach the same distances as our standard nozzles but with with 30% lower flow rates. That in and of itself would have raised your operating pressure sufficiently to solve your original coverage issues - I think.
If you want to contact me directly, please email me at email@example.com. I would like to help you solve your issue.