My speaker grills are maybe MAYBE a human hairs width higher than the screen. We wound up with 4 lines, and all got the m8, and some of the other ones have more noticeable differences in height, but none of the other people using them have said anything, and I don't think they really care. I think that's something we often lose sight of, is that for most people that's just something they don't even think of. Sure, it would be nice if it was perfect on every device, but as long as it's within tolerance and the grills aren't popping up like some people have had, most people won't probably notice or care.
In any case, compared to my M7, the height difference is MUCH improved. My screen was decently higher than the top speaker grill, and that annoyed me as I always felt it when I swiped down the notification bar, but I got used to it and with a screen protector it just blended in to that bump.