if it's overheated it would have allowed the pistons and bores to expand, this has possibly forced the oil from the bores a little and the pistons rubbing and wearing.
it's a bit worrying as i also have wiseco pistons (oversized) however i don't suffer any piston slap, only just reaching a thousand miles but been running on power well.
1-1.5mm is well over tolerance and may be worth replacing the block as a precaution if the pistons can be cleaned up.