ute or sedan?
not sure bout guard rub but the ride will probably br more rigid! jst like lowering any other care!
warrenty........as far as i know, any changes from stock are a problem. but it's totally up to the dealer to make the call as to whether your warrenty is still in tact or not! depends on the individual! be really nice but firm. i got a diff replaced under warrenty even though one rear tyre(the one that spins!hehe)
was totally bald!.......have fun!