I've used GPS tracking to look at out-and-out boat speed (mainly in the lighter stuff when I usually struggle against oher single handers). It does assume a steady wind speed so you have to be carefull. But you can (with care) user the GPS to spot small changes in speed as you change something.

Like say finding the optimum mast rotation.

Sail on a course, set the mast rotation at max (or min) and off you go. This is a datum. Move the rotation a little in one direction in steps of say 2 cm of rope (or something that makes sense) so the mast is moving a small amount at a time recheck the GPS of change in speed. Hopefully you will find a place where the speed is best. Now repeat the example above "around this point" to fine tune it. Flat water helps as this gives you more consistant results, but also may hide the fact that you actually need more power (in the mast) to punch thru waves etc.

Repeat this for the various wind settings where you might play with the rotation and this should give you some guide as to where the mast should be.

Take a pinch of salt as this is only give a guide and may not be perfect as if it's gusty or lumpy, you need to look further than just the numbers

Ditto position of the traveller and should you let the top of the mainsail twist off in the light stuff and or is a flat sail faster than a full one.....