Alas the 5W emitters are not available yet.
However I believe that in this particular application the 5W emitters could be expected to be more efficient than the halogen lamps used in emergency lighting.
The reason is that filament lamps show substantial variation in efficiency and lamp life as the applied voltage changes. If you up the voltage, the efficiency goes way up, but the lamp life goes way down. In an emergency light, I bet they design for long life rather than for maximum efficiency. My guess (not confirmed by any data other than familiarity with LEDs and halogen lamps) is that the current 1W Luxeon LEDs will be more efficient than the halogens used in emergency lights.
Additionally, for an emergency light, you don't need _white_ light. You probably want at least some light in all parts of the spectrum so that you can get color cues, but you don't need an ideal white source. This means, for example, that you could use colored emitters like the amber high dome units or the cyan emitters, and get considerably more efficiency than the white Luxeons.
-Jon