Piston slap is a known Subie quirk...Rotella T6 seemed to quiet down the "Subaru Tick" some. But if it's below 50 deg, it's there until the engine temp gets to the first solid mark at least.
I'd be willing to bet the PO420 is in relation to the spark knock. If your cats are clogged...it'll restrict the exhaust flow and cause all kinds if issues. I'd have the dealer do their repairs and see if it remedies everything except the slap.