How does if affect it, I'm a bit curious now...
As far as quality vs compression is concerned, the highest quality normal maps for FS2_Open can be made by converting the height map to normal map in GIMP or Photoshop or whatever you prefer to use, then copy red channel into alpha channel, and green channel into the colour channels (actually, green channel is the only one needed, red and blue can be blank, however I prefer to have all colour channels the same), then save that as uncomressed, u8888 DDS file. FS2_Open shader system seems to read that kind of file correctly, which shouldn't be surprise because basically the channel management is the only thing that separates a dxt5nm normal map from a dxt5 texture as far as I know.
Of course, this does not offer as much performance advantage as dxt5-compressed normal map does, but for WIP normal maps it's a good way to check how the normal map looks in-game, without any artefacting.
