Below is a link to the suzuki website re prop size and also attached is a test report relating to a RC585. The details in the report reflect the performance that I get on my RC585 with a DF 140 and a 3 x 14 x 21 inch prop
My Suzuki - Prop Shop | Suzuki GB Marine
Top revs is 6000 and we have had our top speed of 42knots coming out of the swellies with a full tank and 2 up. The fuel consumption is as per test report. This is on a 2006 engine.
It would seam that you are running a little low on rpm at 5500. I am no expert but it would appear that the prop size is Ok so is the engine at its optimum on the transom. Cost nothing to try a notch higher or lower just the hassle of moving the engine.
I have been tempted to put a stainless prop on. I had a stainless prop on my old tohatsu but have resisted changing to a stainless on the DF140 after having a few dicussions. If you ding an ali prop it is supposedly more forgiving and as result there is less risk of damaging the gearbox in the lower leg.
Shall we see you in N Wales?