By the time of Pontiac's War, kilted regiments in North America were commonly issued with Indian leggings (for those regiments whose officers cared enough). The full tartan like the one PANGI used as a model for his Highlander were probably last used on the field there on the Plains of Abraham, by the 78th, which is also know to have led the last Highlander charge ever with Broadswords. From then on, kilts were adopted as lighter, cheaper and less cumbersome outfits for Highland regiments, although the tartan would remain as a parade outfit.
For winter dress, records form the first wintering British garrison in Québec are pretty depressing. Highland soldiers were by far the more distressed, being exposed to cold more than their comrades because of their tartan. At that point, regulation did no longer matter and soldiers would dress with whatever was available. Which was mostly covers that would be sawed so as to make pants, and as a result they dropped by tens before the arrival of Spring.
Tartan leggings would become in use during the next decade, and could really be seen in North America during the WOI, not before.
PS: I was being sarcastic about the greatness of imperial troops.
EDIT: @ PANGI
Actually, I do prefer my rolled up blanket, especially for a worn out veteran unit fighting during Pontiac's War (hence the leggings in my version). But if you want to go with the clean and parade field look-a-like of freshly arrived troops still wearing their bulky sword and tartan, then the cover must go
I want to use them both 
You're doing such a great job, I see no point in stopping when you are able to pull out 1st class graphics so well.
Formidable!
... anyway.
