Its been a week nearly since i have converted the twin exhaust to single. I have increase the tail pipe (which use to be the y pipe) by 1/2 inch and exits through a straight pipe. I have installed a second center muffler. Its a hks brand. The noise at idle and cold start is actually not too loud, nice little rumbble. Accelation noise also produces a more defining rumble. No lights on the dash board, mpg the same but low end acceleration is noticeable quicker. Im going to stick it on a dyno to see what is happening. Both stock cats are on so hopefully i have not interfered with emission levels. Declared on my insurance and its cost me £19.50 extra a year.

Be interested to see if Dyno reading has improved much

I really hope there is something.

Few months and the car is running well. Definitely more responsive to the throttle. No faults have come up on the obd (been battling a idle issue but that was a vacuum hose). I took it to my garage who ran a full in depth diagnostic with some good old fashioned hard driving. They reported that all is good. They had 1 pending non fault requiring software upgrade. But no rush to get it done. There are very few awd rolling roads in berkshire to get some bhp results. My mechanic agrees that it definitely moves quicker at the lower rpm range. Luckily though they found the idler for water pump was rattling a bit. So new one on the way.