going on to Suzukis web site the max throttle range is
Full Throttle Operating Range: 5300 - 6300rpm
so there is still some beans left in the suzook which you are not able to find, im thinking that the weight factor is likely losing you 3-4 knots, dump the crew off and try again a couple of flat out runs, as you can do this briefly whilst running in at the later stages
if you were to increase the pitch your rev would go further down the scales with the given info.
Nos is superb with pitches/props would be worth talking with him, or go to Race marine and try a couple of props there, maybe if you have a 19 pitch ally, try a 17 pitch Stainless, also try increasing the size by 1/4 inch on the s/s prop, (eg your running 12x19 ,,,try 12 1/4 x 17 s/s !!