From what I've read and been told, mazda pistons can be made to fit an A15, but you need to deck the block, to get the piston to reach The deck height. (Pistons have a lower crown height) If you worked out how far down the bore you wanted the piston to come, you could deck the block less than what a person building an N/A motor might, and gain lower compression. Although this technique does reduce squish and swirl, it has to be better than using a head saver shim. (I really dont have much faith in their ability to seal under extreme conditions)