Do you have the governored speed of the engine? I really don't know the engine. I bought two of these by mistake. I had ordered it over a 4th of July weekend, never confirmation on the order and ordered again from another company. I thought I might need one for a back up but never had any problems other than routine maintenance.
The good thing about the Diaphragm pump that you can get 200psig and pressure is consistent. I have a roller pump on tractor for spraying orchards. The availability to drive around and coupled with a wide boom on the back lets me spray fields using two 50gal drum
I like the Hypro especially with a walking boom that is consistent for herbicides on lawn and the high pressure for the fungicide.
Have you seen any output on the roller pump using the 2.5HP engine? I've never used one on a small engine, just the PTO's driven. Call Hypro and talk with there rep about your engine and pump before making a decision. He's a knowledgeable guy.
We all are merely gleaners from the masters who were here before us.