Follow up: I got an AMMCO portable hone, and used the 220 grit. Under the supervision of my 4-cycle engine guy (John Kurtzman of Kurtzman Racing Engines), did a very careful hone, getting rid of some depressed spots, and measured about 20 times, resulting in a 54.20R piston size, up from a 54.15.
Reused the old gaskets (since I accidentally ordered MY09 gaskets), and checked the squish – over 1mm, so plenty safe – and very carefully broke it in over a practice day. The engine rocks! My son took pole, and led every lap, winning for the day yesterday.