Having checked the speed of my TIGER since years, the sound barrier is somewhat around 20-21 knots. Even the Hobie trials in the Speed Channel in Southern France have not given speeds over 27 knots which we are sometimes reading.
GPS can sometimes fool you, so it needs more than one proof.
Errors can be reduced by setting up the GPS in a manner that the measured speed needs to be maintained at least 5 sec, so that measuring errors are minimized.

Curious to see your experiences!

BTW: my max speed is 20.3 knots in flat waters of the Danish islands.