The X-pipe is to balance the exhaust flow from both banks of the V8. The idea of removing the back cats is to reduce the restrictions on the gas flow. I found that all the engine braking disappeared after this was done.
There are still 2 cats left at the base of the headers, so MOT emmisions is ok, apparently it has 4 for the american market..
In terms of value for money it was great. My wife thinks the car is broke but she's not much of a petrol head. Going through tunnels or just putting your foot down on an empty road, it brings a smile to my face.
I would like to get the headers done, but the cost means its going to be 2013 as i have some other stuff planned in 2012.