My new 4x4 RS500 shell is seam welded, but it was a group a motorsport jobbie so all the seam sealer and sound deadening etc was missing which made the job a lot easier - its a bitch of a job to do on a road shell as all the seams have to be cleaned out back to bare metal first, and time is money.

Only worth doing if you are going to use the car on track, but it does stiffen up the shell a hell of a lot - I foam filled all the cavities on mine as well so this baby is rock solid unlike a normal 3dr shell which flexes big time.