I didn't want to completely remove the air box in my Supra and pull too much air in from the hot engine compartment, so I just cut the air box a little.  Now it pulls some air from the engine and some from under the head light. 

The K&N is a direct drop in replacement for the stock paper filter, so that part was easy.  The cutting of the air box was also pretty easy. 
 

1- Since I cut the air box, this is another mod that has no easy way back if I messed it up.

1- Now I have seen where people have removed the air box completely and installed a metal shield behind it to keep hot engine air out.  This is the route I would recommend and I will probably switch to this when I finish the cool air in mod.

As I did this as part of a larger repair as well as a few other mods, the payoff is hard to judge.  If the K&N claims of up to 15 horsepower are true, then this was a great mod.