Update on my last post. I took my car for a long drive on Sunday and unfortunately, when under load on a slow incline, there is a drone. Certainly not as extreme as with the performance exhaust however a drone none the less.
My advice to anyone that is planning on doing this mod is stop at 15mm - possibly go to 18 or 20mm but 25mm is maybe too far.
Unfortunately in my enthusiasm I went from 15mm to 25mm without road testing so I can't say whether a middle point might have been better. What I can say is that I am now about to change my exhaust for the third time. Anyone passing through Kent this weekend is welcome to stop by and offer their moral support