Cat Time

[Fin]

Last year, I just about scraped my MOT, due to emissions. So, it's soon to be time for a new CAT.
I know I can pick up an OEM for around £140 ish... I believe they kick at about 300 CEL ?
Is there any advantage to getting a lower CEL CAT? Where would I look for them? And how much do they cost? I have a stainless steel A pipe knocking around in the shed somewhere, and I am running a Proflow exhaust (if that makes any difference).
Would a bolt-in one be worth looking at cutting into the A pipe? ( I know stainless is a pain to weld, but I know a chap who can practically weld anything).
Pure road use (occasionally quite "spirited").
Thanks as always for advice   s:D s:D