So on my 03' WRX I have a HKS dp catless to a magnaflow staright through muffler. It is all 3 inches all the way through btw. I bought the car liek this but it came with the stock axhaust also. I was thinking of putting it back on just to have a quiet car to drive around (it's pretty loud) for a month or so. Here's the questions: Is is going to be dog-slow and can I mate the HKS dp to the stock exhaust?