performance filter is a pos for not much money, not sure what the gain would be, not much.
Induction kit would be the best way to go. Have a search on the forum, there's loads of different threads to read.
then it would be the exhaust system next
cat back exhaust.
You could pick one of up in the buy and sell section for half the price of a new one!!!!!
so keep your eye out for peeps selling
next would be remove the cats
the IS has 2 pre-cats and 1 main cat
The main cat is located in the mid-section of the exhaust, if you've got a local tuner by you or a competent mechanic that you trust, a de-cat shouldn't be a problem All this involves is cutting out the cat and replacing it with a straight through pipe, which is flanged at both ends so the cat can be re-fitted for mot's. All this can be done for well under £100
ok I would say to do the above would cost somewhere around £400- £500 depending on the induction kit you choose
pre-cats
the 2 per cats can be removed in the same way as removing the main cat. just read through the IS200 forum's should de a thread somewhere
the other option is a new manifold check the buy and sell or buy a new one, up to you
hope that's of some help