I'd second the above, get a 2.5" cat and a custom downpipe made up. All I would say is that you get what you pay for with a cat, the nemisis cat you posted has a crummy ceramic monolith thats more restictive than the more expensile steel type, and it dosent say weather its 200 cell or not either....I'd be a little sceptical....
If you want a genuine advantage you should spent the money on a decent cat such as the HJS unit regal sell or equivenent, for about £300.......any exhaust manufacturer worth there salt would tell you the same.