I think, for authors who are newer and/or authors who don't have much visibility in general, KU can be very lucrative and a great way to get in front of readers. The stuff under what was my "bread and butter" pen name that I use with my publisher and for self-pub contemporary romance always did fairly well (i.e. release weeks, new books I self-pubbed hit the top 5,000 overall, and my books with my publisher would hit anywhere from top 1000 to top 100) but my paranormal romance Chloe Cole pen name was in the crapper and all but defunct (I used it back when I first started publishing about six years ago, and hadn't released anything under that name in a couple years). I got rights back to those novellas, re-wrote and then self-pubbed them. They didn't do well comparatively speaking, moving maybe 5-8 copies a day per book across all vendors. I moved them into KU, and they did marginally better. Then, a few other opportunities came along in PNR for me (I was invited to be in a box set with other authors), so I decided to make the most of the exposure, roll up my sleeves and start writing under that name again as well. THAT'S when everything changed. I've released probably seven novellas under that name since November and sales (and borrows) have since exploded. That pen name is now my bread and butter. February was a brutal month whereas, prior, I'd experienced 100% growth every other month, it was a total flatline and even dipped at points, but March things are back and popping again. Yesterday I had my highest borrows to date with 278 for the day (and almost 200 sales). That said, I don't think moving stuff to KU in and of itself would have yielded the same result (or, it hadn't when I did it initially). There were a lot of different facets to how it all happened. I did lots of keywords testing, changed most of my covers out, began using new methods of advertising, got into several private groups with like-minded people in similar genres who were also gung-ho about marketing and what not. I also watched trends and wrote to market and really tried to capitalize on the pro-short form aspect of KU.
So, I guess my advice would be this: If you have poor selling books OUT of KU, just moving them until KU will not likely make too much of a difference for you. If you move them and then try to capitalize on the move, write more in the series or a connected series etc, then yep. I do think that could work, and there are definitely authors making it work to their advantage. Good luck whatever you decide!