I agree, there are aren't enough shooters in the piston class at GPs at present to offer trophies to each grade winner, and that the current trophy distribution should remain.
At present, if you shoot in the piston class at GPs, you shoot against the best in the world, which is no bad thing, but if there were grades, then maybe it would encourage more shooters into the class.
I believe there would be no AA shooters based on the current grading system in the piston class, so that gives everyone something to aim for.
So to summarise, grade piston shooters on the current grading system. Because of the low numbers, give trophies to the highest scores only.