I lean towards outlines on the balloons myself; I like for the words to pop a little from the art since I feel like it leads the eye a little better! But that's personal preference. : ) Though balloons with no outlines can get a lil awkward over a white or light-coloured background.
As to the highlights layer.... I know this is kinda semantics quibbling, but to me, this isn't a style change! This is just "should I add this glowy effect or not." I guess the thing is that I really don't think this is a big deal -- it obviously has an ever-so-slightly different look with that layer there, and it looks nice, but it's fine without? It's about whether it's worth the time to include it to you, based on how it looks to you.
For me, which overlays and highlights I add to my pages vary a lot from scene to scene based on the lighting, so I don't think it's super important to keep the same effects on every single page!