The narrow optic will NOT increase the Lumen output but rather concentrate that total amount of light into a tighter beam. This will create a beam that will more brightly illuminate a surface at a given distance although the illuminated surface will be smaller.
It is exactly the same as the "focusable" Maglites. The output of the bulb doesn't changed but the light is concentrated or spread out.
I have found this page on Wikipedia to be very helpful regarding the different terms and units:
http://en.wikipedia.org/wiki/Illuminance
The information provided on the RSMK website http://www.em-mgt.com/LED/Peak.html is good but does create some confusion since Curt seems to use the units Candela, Candlepower, and foot-candles interchangeably. The units Candela and Candlepower are equivalent and measure Luminous Intensity whereas foot-candles is a unit of measurement for Illuminance.
These measurements are concepts that I'm still trying to understand well enough to explaining clearly and I don't want to confuse the issue more right now by trying to give my explanation without some careful thought. I mention them because there is frequent confusion on this forum (and in general I suspect) regarding Lumens, Lux, foot-candles, candlepower, whatever.
To possibly address the core of your question, if you shine the narrow beam light at a surface it will appear almost twice as bright as the medium optic (based on the 85% value quoted) but (or actually because) that bright spot will be ~1/2 as big (exactly 1/2 the area would give a 100% increase). Remember this is area not diameter. The area increases with the square of the diameter so a hot/bright spot with 2X diameter will have 4X the area.
It would be very helpful if all the vendors measured and reported (truthfully and accurately) both the total output in Lumens and the (pick one to standardize on) Luminous Intensity or Illuminance. If 2 lights had the same output in Lumens but one had 2X the Illuminance then you would know it had a much tighter primary hotspot. You still wouldn't know much much/anything about the spill but it would be a start. Likewise, if 2 lights had the same Illuminance (foot-candles) but one had 2X the output/flux in Lumens, you could surmise with some confidence that the hotspot was ~2X area. Of course reflectors/optics with very smooth transitions and the like make this comparison more complicated which is one reason why people want to see beam shots.
I hope this was helpful and I didn't go overboard here.
Regards,
Rich
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