Seriously, ive had high flow cats put on both my 350 and 370, i agree wholeheartedly with tarmac, get professionals to install it.
Its a very tricky job, especially going in through the engine bay. Also if anything goes wrong its up to the garage to fix it. For the sake of 50-100 quid its a far better option.
agreed - if you have the option
but it is definately a do-able DIY job (with the correct tools)