I was originally booked on the July1st departure for that trip. But GCT called me a couple of months ago (maybe more than two) to explain that they'd had to cancel the Scottish Highlands extension on that trip because they didn't get enough people. Which may, or may not, be true. In any case, they offered me a change to departing a week later which I took. I really want to see that extension and, since I travel solo it's the only way for me to do it.
If I were in your situation, I would certainly do it on my own. Check out Trip Advisor for hotels, restaurants and tours. Look at the reviews from people who've taken the extension and see what they have to say about the "must see" and "waste of time."