Have actually tested this yourself, if not, what are you basing this on? The Nexus 6P does go into a low-persistence mode (sustained performance mode works on both). As far a laggy experience the demo I created seems to run pretty well on a variety of phones. Plus, have you forgotten the issues of GearVR users who have a Note 4? Many of the newer games and demos do not run as fluid as those on the S6 and S7. So the same case can be made for GearVR if you don’t have the latest phone your are not going to get any of the new features like Vulkan, multiple pipelines, faster performance.
I have both and have developed on both. My demos on the GearVR and Google VR run about the same, close to 60fps (yes this is the S6 and the Nexus 6P). It should also be kept in mind that Google VR is not done, nor are the plug-ins for UE4 and Unity 3D. Also, Samsung will support Google VR, I believe their name was listed amongst the phone makers - starting with the new Note 6 rumored to have Android ‘N’ when it ships.
So yes, I agree you will not get the performance with phones without the proper hardware hooks and newer SOCs. But this is true for the GearVR as well. However, simple games and demos can now be done in UE4 without having to buy a GearVR or a new phone. This is a big win in my book for the largest audience without having to use Unity 3D.