it would be a problem for any extreme or custom made stuff, but for different wheels, springs, and other basic bolt ons it shouldnt be an issue. hell, i see dealerships around here passing out leased cars that are already modified. you know, you drive by the caddy dealership and see an escalade on 24z with billet grill etc for sale, you can lease those too.
SEMA has excellent information regarding dealers/warranties etc on their website. they have spent years pushing legislation to deal with bs like that.
ive seen people cut coils, hydros, graphics and shave door handles on leased cars. they said they would just return it to stock when theyre done having their fun. they probably didnt take it to the dealer for service though, for reasons that sigma pointed out.
