I didn't notice I'd slipped in the K11 section, sorry for the intrusion, I must have hit the new posts button. still, an interesting conversation to be had so if you don't mind the new fangled K12 driver sticking his oar in for a few moments more.
Seat of the pants dyno has always been a thing, but you aren't going to notice 3hp loss really (bit like end of a year before a service where you can be down 3-6hp because th air filter's full of smut, the plugs are coked up and the oil isn't working as well).
It might change the power curve slightly, most things you do to the way an engine breaths do. this is what you will notice from your arse in the seat, but over the entire curve on a 0-60 run, it's so small as to make no real difference. without tuning to match though, all you are really feeling is a slightly quicker uptake in the revs, this doesn't mean more power though.
taking air from straight behind the grill is a good idea, it extends your flow path to the throttle body which can have detrimental effects on the flow if not managed correctly still better than dirty hot air from in the engine bay though.
still not convinced it's worth it.
K12 driver interlude over, thanks for the chat.