i did ask whether it would be possible to mate a miltek or apr DP to the standard exhaust but was advised that it would not be a good idea because the flow would be better but the standard exhaust would throttle the increased flow , shame really cause i know a lot of the ST boys have done this on their focus with good results.
I can see what they are saying but i did it and still got good results/gains. Not ideal but i didn't have the cash for the full system and used to have the dual pipes.