I did the Big air box mod. I found that with the stock filter it doesn't inhibit any ill effects as far as rideability. There may have been a slight loss in low-end TQ but you go through it quick enough not to really care. Then again, I'm up in high altitude territory and need all the air I can get.
I'm going to try a Race BMC filter and airbox seals to see if it helps in HP and then see if it turns it's ugly head with a dip in the 2500rpm area as it is notorious to do so.
I'm sorry. I didn't mean the Full box mod as far taking out the entire floor. I cut out where the filter sits. That's all. I know some people call that a Medium mod. The flapper removal is considered the small box mod. The Full box mod is the removal of entire floor requiring the use of a piece of foam cutout to cover it up.
You need to know that the BMC race filter requires the medium airbox mod. That is the one where you cut out the flapper valve and the surrounding plastic to the ridge on the airbox bottom. The ridge is the problem. The race filter passes more air by using deeper pleats (more surface area). The outside diameter of the filter is stock but the inside is smaller. It is not big enough to fit over the plastic ridge on the airbox bottom. The ridge must go. I cut the bottom of the airbox just barely outside the ridge. That leaves enough base to seal against the filter gasket.
KS has great pix of the base. He cut out the ridge too. What you end up with is the air hole that is the size of the inside diameter of the filter. That is as little restriction as is possible without a different filter. All that stock plastic is gone! Where you lose HP is when you crank the throttle at low RPM and the RPM is not yet speeding up. The flapper was acting just like a slide in a CV carb to modulate the airflow/mixture as a function of RPM. Without it, the instant throttle opening is causing the injectors to squirt but the air velocity is not increased yet. That is a rich condition until the RPMs pick up and dilute the spray. I have noticed a very slight change in low end power when wrenching on the throttle. It is not much and is overshadowed by the potential high end power improvement. Of course, the ECU will have to be tuned for the new mods to enjoy that power increase.