found the issue, tried 391 driver and got SMI to work properly. Going to try updating from that driver see what happens.
Edit: updated driver and back to same old thing. WDDM not working. Assuming somethings going on with newer Nvidia drivers. Going to try older 400 series driver next see what happens.
Edit #2: Seems 399 is working for SMI. for P104-100 I haven't seen any difference in wattage using Inspector vs MSIafterburner. at best 3 watts maybe?
Can confirm that drivers 399.07 and WDDM mode has made Inspector see P104-100. I managed to lower each card for about -15watts compared to Afterburner. Most cards work at 700 mV.
Glad to help! However I noticed serious hashrate drops when lowering voltage using Nvinspector around 1500-1600mhz is needed to achieve full hashrate with P104's. I've had mine Overclocked to +800 on memory +150 core @70% power on MSI afterburner. They're doing about 40-40.5mh/s @ 125w
P106s will more likely be different. Those should do about 65w-70w on ETH-hash.