I only did one such test a few months ago with two versions of the same squeeze page. The benefits of opting in were written in a hype form on one version and a non-hype form in the other.
The non hype form sounded like: "You'll learn a proven technique to significantly boost your confidence in just a couple of weeks." Which in my mind was a realistic and truthful promise.
The hyped up form sounded like: "You'll learn a secret techniques that's guaranteed to make you super-confident in 24 hours or less, no matter who you are and how low your confidence is right now". Which is a very exaggerated promise.
I was pleasantly surprised to see that he hyped version performed only slightly better than the non-hype version. Many of the people who opted in were probably different, but the optin rate was pretty close.
Which was encouraging for me because it showed me there is plenty of room for honesty in marketing. But I don't know, maybe they split-test wasn't conducted well, or maybe the conclusions don't apply in too many niches.
That's why I'm asking: what are your experiences with this?