yeah you will loose a few horses. the reason for this is down to air being slightly pressurized in the OEM intake system, this provides a slightly pressurized environment. where as the K&N apollo just doesn't for obvious reasons, as it's route is atmosphere directly to engine near enough but it's not something you'd notice on day to day driving. the main negative is that you will create flat spots in your rev range (this is actually what the OEM airbox does, gives you the optimum smooth RPM range, due to the shape and flow of the OEM system) but again that's hardly noticeable.
the main thing about the K&N Apollo is that it sounds AMAZING