I cover the speed readout with tape for racing and just use the lift / header readout. That makes it legal.
Having used it for a while, my personal belief is that it is a good training tool, but of marginal use while racing. During training, it can help refine what the boat should feel like when at full speed and sharpen your senses of when the boat is slow.
During racing, a TackTick compass is far more use.
I'd recommend that the class permit the use of GPS speed readout during racing. It will not create an uneven playing field and will certainly not create an "arms" race where everyone is forced to buy it. GPS speed readout simply is not that valuable during racing for a boat like an Aero.