I don't think there should be an issue in making F&F and Kong 4K. Nowhere does it say that 4K projections HAVE to use the Dolby 3D projection system that Transformers and Minion Mayhem use. Dolby 3D is just convenient because it can technically use just one projector to project both the left and right eye onto the screen. With the polarizing 3D glasses system that Kong (and the other older attractions use), it requires one projector per eye. But I don't think there would be an issue in upgrading Kong to 4K and still keeping the cheap polarizing 3D system. Hopefully they'll make Kong brighter as well.
I'll tell you one thing though: F&F is gonna feel a lot more realistic than Kong. I can almost bet you that the ride will take place at night for the drag race. If it's at night, the tops and bottoms of the screens will blend in better with the "night" look of the scenes. The opposite happens with Kong. Since Kong takes place during the daytime and you see the bright jungle and everything, you can clearly see where the screen cuts off on the top and bottom. But F&F will probably solve that. (Same reason BTTF always felt more realistic to me. Whole ride took place at night, which made the black blackgrounds blend in with the sides of the screen)