After chatting with some of the knowledgable 510 folks on the bluebird list, it turns out that I may have an inexpensive solution available. Instead of spending $160 on another set of pistons, it looks like I can take the pistons I have and have them pressed onto a set of spare shorter z22e rods I've got sitting around. That'll lower the piston height by a full 3mm, leaving me with 0.45mm over deck. That's still a bit too high, but I can hand-sand the piston down an additional 0.2mm to bring the piston down to something reasonable. Sounds good to me!