Paul,
While the undertank K&N may pull cooler air, that air has to travel farther than slightly less cool air going straight into the throttle bodies (if in fact that air is cooler). Also, I think you gotta consider the amount of and speed of the air that the undertank allows versus putting filters directly on the throttle bodies.
Don't know enough about physics to know if it's the coolness, the distance, the amount, the speed, etc., but a glance at HP/TQ numbers throughout the various forums will confirm that setting up filters directly on the throttle bodies is the best way to go so far as performance.
I've got the undertank and it's much better than stock, but I don't plan on stopping at the undertank. Right now I'm just trying to get the most out of my current setup and learn about Tuneboy.
Tall Tx Guy