I have fitted loads of UFH for my self and big firms. All of which has been designed and supplied by the manufacturer. None of which have ever been supplied with metal plates.
In fact I have just got one up and running last week, the temperature on the mixing valve on the manifold is set to minimum is set to 40 degrees (can go to 85) and te people are saying it got nice and hot really quick.
That could well be the case mate. But you can't randomly advise someone that their DIY UFH system will work, you or I clearly don't know the property or anything about the construction, U values etc etc.