i wouldn't run any unfiltered air into the airbox. if you want to run a lot of air into the engine make/modify an airbox as you see fit. i did one for an 888, it's a bit different to the 851 box, but the basis is the same. the std 888 box runs the two tubes into the front, but it's all filtered on all models.
this one was done because the bike had 996 style throttle bodies bored to 54mm and huge trumpets that had been modified as they were hitting the std air filter. i replaced the filter with k&n pods mounted on modified muffler end caps supplied by a local exhaust maker sitting between the trumpets and throttle bodies. but then one of the trumpet mounts broke off the throttle bodies so we replaced them with original sp5 throttle bodies from gowie, making it all a bit of a moot point. but the work was done by that point.
i just cut the front out of the original airbox and used a piece of rubber sheet to seal the top of the radiator back to the airbox so it wouldn't pick up hot air. it still ran the tubes from the front into the sides - you can see the oval holes at the front top of the sides.
not sure what it did for power output, but as a 955 with some 37/31 valve heads from germany, g inlet and a exhaust cams and a termi spag system it made the same power as a 996sps. went well once it got itself together.
