Even making an exception and raising the limit on the A9 trucks would still be travelling slower than they do anywhere else and people will still overtake them because of that so I'm not entirely sure what the trial can hope to prove. You'd either need to go the whole hog and raise it to 60 or drop the limit for cars down to 50.
A 50 limit would half the differential between Tesco HGVs and cars at the legal 60mph and thereforen queues would take longer to form and drivers would be happier to sit at 50mph than 40mph in a queue. The number of reckless overtakes would hopefully be reduced.
After all 40mph is a speed limit used on many urban roads with frequent junctions, many cyclists, and footways with pedestrians. It's appropriate for roads the standard of the A9 in rural locations to have a 50mph HGV limit.
Average speed cameras? I've no objection. I drive motorways all over Scotland and the M77 is the easiest. Traffic bunches less and there is fewer speeding vehicles. That said I don't think the average speed cameras will do anything for dangerous overtakes. The difference with the A77 is that cars can pretty much drive at the legal limit. On single carriageway sections of the A9 drivers will still want to overtake if the are forced to drive for many miles at 20mph below their limit. I'd rather the cash for average cameras was put towards more dualling.