I noticed a little bit lower reception with the SE compared to my 6S. Not drastic, but still less. In some areas that can make a difference, such as my office!
So far I've not noticed anything, but I'm not really paying attention and where I live, I get great coverage, i.e., Boston. I'll be going up to NH this weekend, and that's a particularly weak region of ATT, so that will be a good test I suppose.
Yeah, I thought about going into field test mode and reporting back the numbers, but then I decided to finish season 3 of The Newsroom and take my daughter to watch the high school girls soccer game instead.