A few years ago, I returned my early-2011 13" MBP because I was convinced it was overheating. Swapped it out at the Apple store without any hassle. They even opened the box with me there. Took it home and subjected it to the very experiments I performed on the previous one. Lo and behold, the temperature ran up on this one too and I discovered there was nothing wrong with the other one, just the Sandy Bridge MBPs had a tendency to run hot. I felt guilty for taking advantage of Apple's lenient, 14-day exchange policy and I sometimes imagine my beloved computers as having a silicone soul. The one I replaced just wanted to be my companion for years to come. It must have become a refurb when it was deserving of a finer fate.
What drove me nuts, for a matter of months, was that I believed the one I replaced had a quicker stock drive than the new one. Whenever I clicked documents or downloads, the icons within the "stacks" would pop up faster on the old computer, whereas the new one would show the placeholders longer. Probably my OCD at play but it goes to show you may find trifling things to fret over with a replacement computer, which may lead you to regret having swapped out the original.