I'd say that's a moderate amount of stretch. Stretch is obviously a function of how often you wear a watch and, more importantly, whether you wear watches tight or loose.
I wear mine tight so stretch isn't a major issue.
I wouldn't worry. It still looks wearable for a while yet and, if you wanted, you could get it "seen to" for about £120. As long as the clasp is as secure as ever, I wouldn't worry about it.