That is because sensitivity is not measured on the BA driver, but outside the nozzle of the earpiece using a dummy head (or something similar). Given that the voltage and power doesn't change (standardized tool), putting a denser filter means it is more difficult for the BA to push air from inside the BA, pass the filter and out to the coupler (where the measurement is taken). If you cover extra fabric over your car speaker, volume will drop as well (same watt generates less volume) - it is the same principle here. The sensitivity isn't measured on the speaker itself, but in the seat, so to speak. There is where the acoustic impedance of the whole system can be changed by filter (and I am not saying the sensitivity or electrical impedance of the BA itself have been changed).
As for whether the top-end will remain the same, that will have to wait till some one who own the PFE has compared both. My personal guess is the top-end will probably be a little weaker, but still more than enough to keep thing interested. Well, that's my hope anway.
