Not all forged piston motors slap. It depends on the piston pin offset, piston material and piston-to-wall clearance used by the builder.
Then stick to the lower expansion 4032 pistons (Mahle, Wossner, Cosworth) and pick a builder who is used to setting the clearances appropriate to your use. For reliability, blueprint the bearing clearances. Subaru doesn't do a great job in making them consistent from the factory which will affect the oiling, especially on the #2 & 3 rods which are fed by the same main bearing. I have 5 blocks sitting here, all of whom failed on either the #2 or 3 rod.