Gilo, you should be able to correct the spikes on GPSAR software, to give you a more accurate 10sec average.
I agree with Mitch here, once you get up to speed the drag of the rig comes into play, probably more than any scratches or dings in your hulls. Lose the spinnaker and go 2-sail reaching (or 1 if solo), should give you your best speeds as there is less drag.
The mathematics and engineering gurus might elaborate on this, but as speed increases by a small amount the drag (hull drag & foils, as well as the rig through the air) increases by a much larger factor, so you end up throwing much more energy into small gains, at the top end of your craft`s speed potential. It may just be that your boat has a max. speed potential of approx. 21knots, which will take the perfect conditions and setup to acchieve. This means you are, at 19.3knots, operating at 91% of potential, and any small gains require a large reduction in drag, so you would need different naca section foils to acchieve this, maybe smaller daggerboards and even smaller rudders.
My experience in this is from windsurfing, where a fin size change from a 34cm fin to a 32cm fin can give an increase in top speed. I sailed on Saturday with a 9,5sqm sail in 12-14knots, and although I had enough power to plane easily, felt slow - then changed down to a 6,6sqm sail and, although seldom had enough power to plane, definitely felt faster when planing (no gps).Although I had less power, I had reduced the drag by a greater factor. My all-out top speed is 32knots in strong conditions (approx. 30knot winds) yet I can easily go upwind at 27knots in 18knot winds. An increase in power will only help if you can fully harness that power, which in 20knots of wind you probably can`t, so you will be increasing drag with that increase in power.
So, smaller daggerboards, smaller rudders, smaller sails, and make it plane