Prop size

[Joker2]

I have a 75' V20 1995 150 Ocean Pro ( rebuilt completely in 2010 ) The boat has a 15/12 X 15 Prop on it. After breakin it in last season, able to open it up this year Ifound out I was only getting 4500 RPM's, had the Prop reconditioned for more RPM's, not sure why but now when idling along or going below 2000 RPM engine vibrates I even feel it at the wheel when I bring it up I can't notice anything, I might be crazy but could the prop be out of balance or something like that. Also I have my doubts about the mechanic who sent out the prop, it came back painted but they just went right over all the old peeling paint, no sanding or anything, thought that was lame. Makes me wonder what kind of job they did on the prop. The prop guys are a major prop house in this area. Is there any way to find out what I need to know without using a known good prop?