By way of anecdote, I drove (yes, yes...) up to the Strathpuffer last weekend and the A9 was basically deserted north of Pitlochry (as is almost always the case). The existing levels of dualling offer ample passing for cars vs lorries and, of course, the average speed cameras have vastly improved the safety of the road.
It is noticeably more 'chilled' compared to when I spent my RAF career (2000s) driving up and down the thing to the Moray Coast, via Grantown. I holds my hands up, I was one of the worst offenders for flying past big lines of traffic in my youth in my fast car.
There is absolutely no business case for dualling the A9 (let alone the A96). It is simply not busy enough and the destruction of the Tay valley would be environmental vandelism, especially in the current 'climate energency' rhetoric. Just looking at the destruction of the area north of Perth that has recently been dualled.
Of almost more impact than the dualling is the requirement for grade separated junctions that dualling brings. Enormous bridges for unclassified roads going in all over the place and it's only just begun. Total waste of resources when sustainable infrastructure could be delivered with that money.