Stock pistons are fine, until you get knock. So if you are experimenting, raising boost, tuning with bigger turbo etc and you know you ran into knock and cracked a piston, then oem could be fine if you correct the error that caused knock. BUT, if you are rebuilding the engine before modifications and you know you will be doing some tuning with bigger turbo etc you will be better off replacing the pistons with forged ones to add safety as stock pistons don´t allow much or any error.