Um, isnt that the O2 sensor? If it is, you do kinda need that!! And the GTS-T and GT-R cats are completely different, the GT-R cat is the best you can get, better than most aftermarket cats, which is why they are so expensive. If you find one, buy it!! Can you extend the wires by cutting them and soldering a new section to them? Make sure the solder is of decent quality and covered. Otherwise, gut the cat and weld a pipe through the centre. I would never do it, but you might get an extra 3kw!