The thing with King is that he was never going to change the biggest factor in his draft stock: speed. Sure, he ran a 4.5 at Iowa's pro day, but that still didn't stop him from falling to the 5th round because teams were still worried about that. I highly doubt the situation would have changed had he left a year earlier, considering he still projected as a 1st rounder throughout the 2016 season. In fact, he may have been taken even later, as one could argue 2015's class of DB's was deeper than 2016's. Fortunately for Desmond, he's showing why that was a dumb decision with his play for the Chargers.
As for JJ, he has the raw athleticism that the NFL wants and needs in CB's right now, and the man-to-man skills as well. He could work on his zone coverage and physicality in the run game a little, but honestly, I really don't think he has much to gain from coming back. I think it will boil down to whether or not he wants to finish his degree, and if he wants to finish his time as a Hawkeye.