The power map controls the dissipation of the glare on a surface. Take a look at the first 3 bands in the angled picture of the B29 I did.
Note how the "glare" is reduced at each band? The first being 0, which is full glare, the second 128 reduces the glare by about half, and the 255 value reduces it to a narrower band again.
Download that B29 skin, take into the skin viewer or in the game, and move around it using different angles. What the power map does is pretty well depicted in there. I went through this so you guys could see what those maps do and how they play together.
If you are doing a bare metal skin and have not done power and environment maps, then your skin is not going to look very good. You cannot do bare metal with just a normal and specular map and have it look its best.
The anit-glare panel on the P51D is very different. Personally, I would start with a low amount of specular, with a high amount of power, and a touch of environment and go from there. Very different from the bare metal, but without the power and environment maps you are probably going to get a washed out anti-glare panel.