It’s not just theory. IF the factors that I stated are correct, it absolutely works.
Now does sun sleeve X under conditions Y actually have a net cooling effect? And is that effect enough to notice? IDK, that’s where having the right riding conditions (not too humid) and the right product matters.
The problem with a base layer is that it’s not the top layer that actually sees the airflow. So what you’re wearing on top of it can totally cause a base layer to not work. So it’s effectively impossible to say a particular base layer works if you’re not also specifying what’s being worn on top of it.
I’m no I don’t think so - all scenarios are predicated on osmotic(?) pressure. More layers alter the pressure but not the physics.
Where cooling sleeves would definitely work better than none is as posted by someone else - wet the sleeves.
That is generally what in theory means, though, isn’t it? In theory it works, but in practice it has to fulfil certain criteria. I am saying, I am not aware of these being fulfilled.
As I said, for me sleeves only work being cooler is when I wet them.