I've read about a lot of different opinions as to what the ideal cat-back for an '07 BF MKII.
Could anyone suggest what the best diameter etc. would be if I had a set of extractors fitted also, for best performance and sound.

Brands and links would be appreciated. Thanks.

Take a look here venom-exhaust-works-f74/end-of-year-fordmods-special-on-all-cats-t97858.html and maybe post a question in that thread. This person is a Sponsor for this site and I am sure he would have some good ideas and prices.

nothing smaller then 3''

Keep in mind mate if you change the cat or possibly even put extractors on your check engine light will come on because of the confused reading the second o2 sensor is reading from the extra gasses flowing. The only way to remove it is via a tune. I have an extra piece of piping that mine plugs into and this kept the light from coming on for a good 8 - 10 months but its been on and off the last 2 months for me, doesnt bother me though as i will be getting a custom tune in the new year