Excessive bump mapping can look a bit weird from the pilot's seat in AH2. The highlights and shadows moving around on the top of the wing as the plane manoeuvres do not look realistic so I keep this effect turned down on AH2 aircraft. AH3's normal mapping does a better job in this regard so I make the effect more visible there. GVs don't tend to roll around so much and have much bigger bumps so the bump mapping effect is much more useful there.
Specular mapping is much more useful for planes, particularly bare metal planes like your P-51s. I can't say I have noticed any reason to adjust this effect for distance though, it works the same up close as further away. It is just a question of how dull you make the painted areas and how shiny the metal areas and that varies from skin to skin. You just have to compare your skin to WW2 photos of similar aircraft and ask yourself if it looks the same.