any chance the slider pins/bores on the calipers are worn beyond useable service?
I haven't tracked too many single-piston calipers, but our LeMons Miata never has a problem with pad taper..
my wilwood dynalites were bad about it, but I would just rotate the pads every ~4hrs of track time and they would wear evenly.

for a Chump race, just flipe them at the end of each race day and voila. ???