I have done the resonator delete and replaced it with an X-pipe up front on my 05 645ci. I kept the stock mufflers for now but I plan on doing a whole exhaust at some point. I will try to get a video and post it up here within the next couple of days. It sounds really good on idle and under a load but without good mufflers you will look stupid reving it up past 3k from a stop. I also lost some low end power but the high end revs nice and fast.