Volvo props.?
 

I have a Volvo 502 with the duo prop drive. When I bought the boat (25ft checkmate) I figured I would blow up the drive and put a bravo on the back. I year later the drive is working great and I'm starting to like it. I have F8's turning 4800rpm's. For more speed do I want to run smaller F7's and turn more rpm's. Or is there some work that can be done to these props? Is there any one who works on the Volvo props.? With a bone stock 415hp(502efi) turning 4800 I can run 68.3 gps every day. My boat is a 25 ft Checkmate Convinsor 251. Is this drive slowing me down or are these good numbers? Thanks!