So far I've installed K&N FIPK & Airaid intake systems, MIT's, Corsa and Flowmaster systems in H2's, Tundra's and the Yukon.
In my experience the perfornace gains were marginal at best. In most cases I didn't feel a lick of difference, except a sweet exhaust note.
However, I do agree that the H3 feels underpowered but then, I bought it for the gas mileage (over our H2) so I wasn't surprised.
If you have any doubts at all but want to "try" these add ons, at least buy it from a vendor that will give you a money back guarantee.
On our Yukon I installed an Airaid Junior kit (filter and MIT) and the Airaid Throttle Body Spacer. Initially I "convinced" myself that my $350.00 yielded more usable ponies but after a long drive up the mountains I came back and returned everything to
http://www.autoanything.com. They were great to work with. I can't believe they took back a used filter!
Don't get me wrong though, perhaps the H3's OEM intake and exhaust is so restrictive that an intake coupled with a free flow exhaust works. But it's an expensive lesson.