Makes a very slight difference but probably the only noticeable difference will be the awesome sound it now makes ;)
The typhoon kits are popular with people on a car forum I use but there was a problem (I can only speak about Corolla's) with holes being blown through the pipe that attaches it.
For the money I would say its not worth it as a standalone mod, if used in conjunction with other engine mods then maybe worth the time/money.
It will also increase your insurance ;)