Barrels seemed have the biggest impact on performance from my observations, or at least on the M416, feels like they do way more than the current description lets on.
Unsure whether you’d count it as making an impact but the LPVO is by far the best optic in the game right now. Super versatile and keeps you honest at all ranges, didn’t come off for the entirety of the beta once unlocked.
Pistol attachments look awful so I refused to use them beyond red dot and extended mag which are as expected. Ridiculous. Much prefer the route some games take which is having different slide options, much more sleek than having a shotgun sized choke for a muzzle.