Okay, so I didn't avoid the 'chuffing' update after all. Even without launching Home it was still applied!!
How did I find this out? One minute all was good with the world and literally the next no lights or display in the Rift HMD.
So I had no choice but to launch Home, set up the Rift again and all was good although my Nvidia Surround desktop was completely screwed up upon restart and I had to reconfigure that.
Whilst inside Home however I did find some High, Medium & Low graphic settings specific to the Home screen only. Oddly I could only access this specific menu with the left Touchpad whilst the right Touch pad allowed access to the general settings and menus. I set to Low which reduced the quality within the Home screen but that was fine by me. I also switched off AA in the separate graphics menu.
After that I launched AC via Content Manager as usual and ran several race sessions, one after the other. (Nissan Primera at Donny with up to 16 AI). I experienced no performance hit as far as I could tell and I even thought that text and digits appeared sharper in car. I switched over to the Porsche RSR and maintained that opinion. Weird and probably just a placebo effect! I then switched to RF2, which launches via SteamVR and again, all was good. Race Room as well.
So have I got lucky or did reducing those Home screen graphic settings to the lowest possible level negate the impact of whatever could affect performance? I'm also running the latest Nvidia drivers, 416.16 which might have helped.