If I recall correctly, Michael Brookes said that they automatically implement a 1.4 x super sample for VR, as recommended by Vive

. If so, the 1.5 super sample will either stack on top of that (1.5*1.4 times sampling), or will improve only by a factor1.5/1.4. From ghcannon's comments I suspect the latter

.
Hmm, that seems to make sense.
But then, the standard 1.4SS doesn't look as good as earlier versions with 1.5 enabled, and generally the image looks blurrier with less texture definition than in the past.
Adding 1.5SS in the past added enhanced clarity as well as more definition for the text etc.
I read somewhere that the HMD Quality slider affects the size of the built in SS. Perhaps the devs need to extend the range of the slider so we can opt for 1.4, 1.5, 1.6 etc so that we can tweak right up, at this level and ignore the SS slider for VR.
but your explanation that we may just be adding 1.5 over 1.4 seems to explain what seems to be happening. We are just supersampling the supersample with little benefit and still experiencing all the performance hit.
Perhaps ED has reduced the graphics quality to meet recommended VR specs of the Rift and Vive. But we need to be able to over-ride this for those with high spec hardware.