you do get weather charts in your briefing package don't you?
Yes. They are a good guide. I have seen SAT changes of more than 20 C with a completely clear weather briefing. When queried the Met service commented that they are forecasts, not reports, and are as accurate as currently possible. They also informed me that they don't include several weather phenomena that are able to be forecast but deemed to uncommon to be worthwhile.
instead of going "into a stall" (a bit dramatically put by you, lowspeed band comes first you know).
I didn't say anything about going into a stall. I mentioned stick shaker, if that is allowed to progress into a stall then there is a basic stick and rudder issue that needs addressing. As for the low speed band....does it really come first? If you are cruising at your Max of FL 380 as you described, and the SAT increases from -56 to -46, what do you think happens to the 'low speed band' (sic)? The answer is ......nothing, it stays where it was at -56 C. So does it really come first?
If tooling around at Max altitude a very clear understanding of what drives the PFD displays is a good idea. In that same situation, what happens to thrust available?
At -56 degrees the aircraft was capable of a 100fpm climb rate, what is it capable of now that the temp is -46 ?