Funny you should mention it... I've been looking into that myself due to concerns about the compatibility of ImageIO with LWJGL3 (ImageIO is an AWT thing apparently, and as I'm sure you know AWT and LWJGL3 don't play nicely together). All the discussions I've found indicate that ImageIO is OK to use with LWJGL3 as it doesn't cause the problematic bits of AWT to load, so that concern is settled.
I'd like to hear someone with more experience comment on the 16-bit color downsampling for textures, but I would think it would not be a problem for RGBA textures (4 channels per pixel * 8 bits per channel = 32 bits per pixel, 24 bits of color = slightly more that 16 million colors - more than the human eye can detect). In other words, if I understand correctly the downsampling will only be an issue if you're using 16 bits per channel - which wouldn't make much sense for model textures.