I have a SC300 and about to rock the single for weight factors. From the X pipe-delete the subCAT and 2.75 or 3in straight back and kicked out to the left. Wonder if it'll be loud? Which side piping should I do to maximize torque and HP? Could the stock exhaust be sold for anything?

 

First there isn't an X pipe nor will you need one. If your NA I would suggest 2.5in piping anything bigger would be way too big. It will be louder than stock, just depends on what kind of muffler you are getting. I just installed a new HKS muffler and piping, go check out the thread. And the stock mufflers could be sold for around 50 I think.