I think I have a slapper.

I do not have a Subaru warranty, but I have a non dealer warranty.

All the parts are covered under the warranty but they will only cover mechanical breakdown. Is Piston slap a mechanical breakdown?

Since this is a mechanical/design fault on Subaru part do I have any come back on Subaru?

 

nope and nope the warranty people will say that as nothing has broken it is down to wear and tear so they won't pay and subaru will just say you should have got it done when it was in warranty.
even the my02 legacys have it coming now but i.m say that that model year doesn't suffer from it and wont pay for it