Hi Tom,
I have never had a failure using cast pistons, but changed to forged as a precaution once I was consistently making over 220Hp atw. I have done a few pulls on the dyno at over 30psi on cast pistons, but wouldn’t want to run at that boost over a race with the std pistons in it.
The fresh A12 will be fine for boost. Though I do like to run about .018” (.006/1” of bore diam)ring end gap when turbocharging an A12. For 15Psi you will get away with a tighter ring end gap than that.
Is the A12a all standard? ie, round port head, cam, springs etc.

Do you have acces to a workshop and a eddy current dyno?