We have a thermometer on the outside of the tank on the opposite side of the MHP. It is reading at 74. The chicks are out playing and eating and pooping prolifically right now. They seem comfortable to run around and have not yet gone back under the MHP for a warm up (just under 2 hours since they woke up). So far so good. Should we start seeing a downside we will definitely let you all know. For me right this moment the ability to wash this mat easily and reuse it for my seeds started in the spring as well as any time I want to grow a little treat for the girls off season is a huge plus.
Oh I should mention because it is plastic it sits on top of the MHP a little more solid than the fabric heating pads do. So it takes less support. We have it sitting on the MHP which is made of welded wire with 2" squares and no other support. As they get bigger we may have to modify, but it is really solid.
Oh in case you all have not notice OEF5 and I are married.