Sorry to somewhat necro, but i figured I could assist hughlb and possibly others looking to do similar things.
Here are some results of me tweaking things to the bare metal livery. Note, this is the .lods shape, so this is how it renders in game.
To get it to render this way, I had to alter the the textures folders under F-5E in the game directory. I extracted the zip for the f5e-3, then renamed the files with _spec to _RoughMet. Then reszip the file and keep the original around to revert if need be. This appears to have "enabled" the model to use RoughMet. After that its a matter of following the description.lua things that Skate has posted in other threads on how to correlate the diffuse (color) texture with the RoughMet texture.
From there, play with the rough met files to your hearts desire to achieve your desired look.