Our latest tests indicate that Google doesn't look at individual websites' CTR but at the overall search CTR. If there is a certain website that gets most of the clicks for a family of keywords, that's the website that will rank the highest. So you need to consider the number of searches for your keywords as the improvement target and not the CTR for your website.
For example, if you're looking to optimize for a family of keywords and each keyword has 10k searches per day but you send 1k clicks to your website, Google sees that your website gets only 10% of the clicks, even if your CTR is 80%+. You need to send 8k clicks per day so that google sees there are 18k searches per day but 8k clicks go to your website, which is about 50% of all clicks. That is the signal that Google is after.