Thanks for all your advise. I was just thinking about if I do all the exhaust system and M6 conversion will cost me the same with selling my 650i and get some M6 similar to mine. But I think it's better to just stay with my 650i. There is none M6 aftermarket warranty so far as I do a search for the last couple of days. I know it's costly for M6 parts too.
I just told my friend a no and joke him that "I will trade my car with your if you can get me extended warranty + maintenance till 100,000 miles as mine 650".