The other issue rarely discussed is that the tests are carried out on radar reflectors mounted alone in a test chamber. In practice you have to mount them somehwere on the boat. The standard mounting on front of a mast will be invisible, at least in theory, to a ship approaching from behind. (The Echomax website has some theoretical parameters.)
Following on from this I wonder whether the Tri-ball type (fresnel lens?) reflectors which mount on top of the mast (and which had a slightly lower performance than the Echomax type in the Ouzo lab report, IIRC) would not in practice give a better all round perfomance in practice.
Regarding the potential for two (or more) reflectors cancelling one another out, I would guess that this is unlikely within the length of a yacht, but I would be interested to hear from someone who knows about the wavelength, etc., of radar commenting on the plausibility of this.