I have. I have an 18 pitch 4 bladed prop that hits middle of WOT range under load, while I have a 19 pitch 3 bladed prop that hits top of WOT range under load. The 19 runs a bit faster. But the 4 blade has more lift and doesn't respond to trimming... and also has a buttload of rake to the point of not clearing a standard Mercruiser trim anode (which is why I stopped using it). Both of those are Piranha composite props; I also have 21 pitch blades for the 3 and was able to quantify it (lower WOT, higher top speed, worked well and was in-spec for light loads without towing anything). Sure, it's mostly for bragging rights, but the GPS speedo is how I was able to quantify that.
FWIW, I have a 19 pitch 4-bladed Solas AL prop that my boss gifted me from his junk pile... not as much rake as the 4-bladed Piranha. Later this season I'll give it a try (and hope the hub isn't spun).
The other thing I use the GPS speedo for the odometer and being able to roughly qualify fuel economy. I have had substantial differences between props, and since I don't refuel on the water, can be useful for planning future outings (namely, figuring how much a difference is made and being able to make the right prop/blade choice).
Not everybody boats the way I do. I don't fish. I don't really cruise/raft up. I'm usually in motion (sometimes towing a tube, and potentially towing a skier someday in the future)... sometimes stopping to eat lunch or take a swim. That's what my boat was built to do. So for me, the GPS speedometer is very useful.
