I don’t know the answer to this but acknowledging
@Ron Ellis point about how you can easily forget how it used to be, I would say that the Karousel improvement was at least as noticeable as going from Lingo 2 to 4 and more noticeable than the Greenstreet. In terms of value for money I would get the bearing done before anything else if I was starting the upgrade path again. Even
@david ellwood waxing lyrical suggests to me (but I cant speak for him) that the Karousel improvement is more impressive than the Keel - and certainly better £ for £. I can’t justify the price of a Keel hence the GS is a perfectly decent alternative for me.
Get the Karousel - you might find you are perfectly satisfied and don’t hanker after anything else . It really is noticeable. Twice I’ve had listening sessions recently playing some of my wife’s favourite tracks and both times she was moved to tears. It really does bring out emotion like you’ve not heard before. And before anyone suggests it, they weren’t tears having seen the price tag