Hi Dave,
I have had high flow cats on my 370 and my 350 they add a nice bit of noise. The also allow the car to breath more easily. Both of my sets have passed every MOT with no issues. I don't know if my garage warmed them up loads 1st but they have passed none the less. Tarmac sportz cats are great.
Some of the American brands do need to be very hot to pass as they don't work to the same emissions standards that we do.