Look into the Deckplate mod and a drop in oiled filter, aFe Power is what I recommended since they made the original TRD performance air filters for Toyota. K&N's take in too much dirt. Those two intake changes will net a combined +18 HP and should run less than $100.

The deckplate mod involves cutting a hole in the air box and installing a marine grade deckplate that is waterproof. By bypassing the snorkel when you're not doing river crossings, you'll gain loads more power. It's probably more than +18 HP for you since that's a long way for the air to travel with a lot of bends. Pop it back in when you do a crossing and the snorkel works just like it's supposed to.