Best RPM for Yanmar 3HMF?

The 27hp Yanmar 3HMF in my 38' 15000lb boat will peg the tachometer at 4000 RPM and just barely reach hull speed (or maybe not quite). Right now I cruise at about 6.5kts @3500 RPM. I'd like to get the speed up to 7-7.5kts and the RPM down to 3000 or ?? not sure. Is that reasonable or possible? The prop is a two blade folder. I couldn't find any markings on it, so I'm not sure of the pitch. I thinking of re-pitching or replacing it, but I'm not sure at what RPM range I should be aiming. I race the boat, so fixed props are out.