Guys, I've been looking for the K&N air filter 33-2116 but haven't had any luck. So I went to the K&N site today and searched and lo and behold, a "new" part number appeared 33-2289, specifically for the 04 GTO.
The dimensions are almost the same as the 33-2116
My local parts place (Murray's) confirmed with K&N the stock replacement number is 33-2289. Released last week (May 17th) I ordered one and it should be in late next week. (or June 8th from what I'm told today) K&N told Murray's, "the part isn't available to the public yet" and they are getting it directly from K&N not their local warehouse. The filter was $64, which I think was a bit of a rip off but hey.
K&N Oil filter listed on the website is HP-1007, this may be the one jaguarcult referred to as causing higher pressure, I'm not sure.