i would be very persistant about it possibly bring a lawyer in to your situation. bumpers arent cheap and for them to do that to your brand new car is totally wrong. plugs will always be there and it will remind you of that everytime you look at your new car.
go beyond that lexus dealership and wriite/call corporate, take pictures and keep records of everything thats been said and incidents. write letters. if it were me i wouldnt stop until the problem is resolved. complain about mgmt, etc... to corporate go to other lexus dealerships maybe they can do something for you.
just dont give up! as long as your in the right.