They work pretty well... I had 12 of them on my old 400EX. Ran them for the same reason...I wanted more airflow, but needed protection from mud/water. While it doesn't flow as much as an open lid, if you get enough vents in it, you'll be pretty close.
I'd recommend putting them on the outer edges of the airbox (not directly above the filter). That way when/if water or mud comes in, it runs down the sides to the bottom of the box, and not directly on top of the filter.
I may do this to my lid for the wet riding days... It looks like you could get at least 10 of them in the lid around the sides and on the rear end of the air box.
I didnt notice any gain at all on my 660. We dyno'd it before and after and didnt even jetted it and it still didnt make a difference. Every bike responds differently to mods though so im glad it worked out for you guys. :thumbsup:
I put some on mine tonight... Going to be riding in some mud/water this weekend and wanted to have some better protection. I ended up putting 12 of them in the lid. None of them are directly over the filter so any water that makes it thru should go down to the bottom of the airbox. I'm going to have to tune my Dobeck for less air flow than running with the lid off, but that shouldn't take more than a few minutes.
I did the same on mine for when i run the lid on, but I also ran filterskins over the lid opening. http://www.pcracingusa.com/filterskins.php They helped stop alot of dirt from entering the airbox. I didn't oil them since I already use a k&n with and outerwear. I just ran them for extra protection on dusty trail rides.