if it's not a dumb question how does the stem nut affect the phone mount or are they two separate things you're trying to do? sorry if I've missed something
I gotta say personally I think these aluminium stem nuts are possibly dangerous. A friend wanted one fitting to their bike as part of an overhaul and I politely declined to do that mod once I'd seen it.
As you correctly say, the stem holds the entire front end on so it's not really an area to compromise strength in any way at all. And yes there are the 3 slim nuts/disks which go on the head bearing which should prevent any catastrophic failure..probably...unless they don't. If the main nut wasn't that important why bother with it at all and why such a massive torque required? idk. Just seemed too risky for me for those puny little threads.
Whilst these ali nuts might be ok I'm pretty sure they're intended as single use items (e.g. for racing) so you tighten it once and the next time you throw it away and fit a new one, which in the biking-on-a-budget community is likely to get overlooked.
My dirt bike has aluminium axles and ali nuts (admittedly not ali->steel but close enough) and has a similar sized nut (24mm vs 27mm for the yam), and that holds the back wheel on (equally critical) and is supposed to be tightened to only 44nm. The yam service manual states 110nm as the torque for the stem nut:
110nm is a LOT for anything on this bike so clearly they mean business. I think the flywheel nut is only about 75nm and the clutch basket is 70, similar Very Large Critical Gears inside the engine are in the 60s and 70s and they're all significant chunks of steel.
I may be wrong and maybe it's totally fine but I have messed up my fair share of threads over the years and to me 110nm seems to be too much torque for too weak a material, in too critical a location, to risk affecting its integrity in any way at all. This would apply
especially if your bike is modified and operating beyond intended parameters. Faster bike = bigger forces, which isnt' the time to weaken any critical connections which hold the entire bike together.
If you do use it, please maybe consider it a single use item and use a fresh one if you ever need to take it off again.
You can save more weight with less risk by going to the bathroom before you ride
