You're right, standard reflectors do vary a lot, so the whole thing can be subjective. The Bowens standard reflectors (which they seem to think are called spill kill reflectors) have a highly polished surface that maximises the guide number at the expense of usability (IMO) and their "Keylight Reflector" is very similar. It's also pretty small, and seems to be relying on its mirror-like surface as much as on its slightly different design to maximise the light. These highly polished reflectors are also pretty useless when used with a honeycomb, because the mirror-like surface creates a shadow effect that can be pretty ugly.
Many years ago, I bought the Elinchrom one, and it was excellent.
The Lencarta one is very similar and like Richard, I found a 2.5 stop increase in effective power at 10', and from memory that goes up to a 3.5 stop increase at 15', compared to the Lencarta standard reflector. Indoors, it's incredibly useful for bouncing off of a ceiling or wall for bounce. Outdoors, I've typically used it to create a shaft of "sunlight" or to change the shaft of sunlight that was already there but in the wrong place, or to create backlighting regardless of the ambient lighting, etc. Often, nothing else comes close, because typically when used outdoors a very high level of power is needed and also the light needs to be out of shot, so the high intensity reflector, often fitted with a honeycomb, is the only real solution.
Mike made a valid point too, a lot depends on the flash head. Bowens flash heads have the flash tube set further forward than most others and my guess (I haven't tried it) is that they have allowed for this in the design of their "Keylight Reflector" and I doubt whether this reflector would make much difference with most other makes of flash head.
