I ve got a k-tec ultra really well made but the standard cat has a tendancy to work loose so it is worth buying the sport cat to go with it however that will take the cost over £500. So for that amount of cash I would buy a custom exhaust from Advanced Automotive Solutions in Newcastle. In hindsight this is what i would have done as the standard cat is restrictive and has poor build quality.
Yozza system is good and there is a good group buy happening at the minute.
Orbsound if you have the monies.