Wether or not the macroclimate has an influence on whisky has always been a subject of discussion. The majority thinks no, some even swear not (Diageo f.i.). Diageo started experimenting with this, they moved speyside casks to Islay to see what happened, yet the results have never been published. Moreover nobody even talks about it now. But, we can all agree that some Islay whiskies have a seaweed aroma to them. Coastal distilleries also have this presence, be it more mild and often subtle. This leads me to conclude the macroclimate can have an influence, but only when the opportunity is drastic, meaning for example very close to the sea and its elements. So arguebly you can say this, and the ppm are two of the things that group the Islays and the Island whiskies to eachother. This is as close as the dreaded term "Terroir" goes for Single Malt.
However, "The Highland whiskies" are way to diverse to be grouped IMHO. Some have this "coastal" influence, f.i. Clynelish, but most do not. Give a Old Pulteney or a Glen Ord blind in a tasting and I dont believe they could tell the difference between a speysider. The speyside whiskies tend to be more refined and in particular floral compared to all the other regions, but this is a matter of copper mainly.
As a result you can conclude the regions are simply guidelines founded on the similarities from the production process and the proximity between distilleries. In addition it seems logical that distilleries that lie close to eachother have a likewise production process.
The highland distilleries are all around the highlands. They are what they are simply because they are not close enough to the other regions. Most of them could easely be imbedded with other regions flavorwise. But amongst themselves I simply cannot see the similarities. It is the one region that can't be defined IMHO. Then again, good luck comparing Scapa with Talisker for Island malts, or Highland Park with Tobermory...