Hi Matt,
I am no expert when it comes to wearing lace systems- been wearing only since this Jan- so take whatever i say with a pinch of salt.
1. Lace front properly glued does disappear quite well. I tend to wear with density of around 65-70% and i dont a have a problem of it being noticed by anybody. i also cut my lace right down to the hairline. I guess with Travolta's pic, he had quite a bit of lace left in the front. There is some truth in that naked eyes always pick up things that even good cameras dont.
honestly, i have been asked a couple of times if i do wear, and it was not under bright light or anything- most embarrassing. But that was not due to the front. It was mainly due to not getting the density right and the same people have also remarked why am i wearing when my own hair(what they had seen me maybe week back) is good. so that made me feel good in way that most times i am able to fool people.
2. Facial expressions might have an effect if you wear your system real low. If you wear it 3 to 4 fingers up from the eye brow, it will not be an issue. Anyways, wearing it low might make some of us look like a ware wolf.
So my advise is , get the density right and go for it. Even if some people still suspect, so what. I know i look better with hair than without. The impression/picture others are also going to carry of me is of one with hair so it's still ok.