They won't. People aren't that clever. They'll just not speed through it. Frequently I've covered vast distances on that road and seen some dreadful overtakes on it due to peoples inability to work out that speed = distance divided by time and that really, its not over 15mins to any dcw section to get past the HGV thats inevitibly holding you up. The increase to 50mph for them will ease that pressure as opposed to being held up at 40, you'll be held up at 50, and if you blast past 1 or 2 in your zone, you may push your speed above 60 and get done. People aren't pocket calculators and will err on the side of caution and not do such bad manouvers.
It's not a road that carries many yobs. Vast tracks of it go through the middle of nowhere, by Dunkeld there are no Perth Yobs and by Aviemore all the Inverness yobs are no where to be seen.
Anyway, I suspect, the real reason behind this is to cut speeding, and see if that saves lives. If it does, great. If not, err, they'll need to improve the road. I for one have never understood how so many come off that road, but there you go. I am all for relaxed driving laws, which this is, the HGV's get to go 10mph faster. Whats not to like.
Dinners, an average speed of 50mph actually isn't that bad. Its a 2hr run. I am sure that beats the train. And the scenery is ok, its a nice run.