I use the normal map far more heavily on tanks than on aircraft, with greater variance of shades in the height map and with the normal generator's scale setting set to 10 rather than 3. Partly this is because details on the tanks are heavier, panel lines are wider, fasteners bigger, exhausts are rusty, there's a lot more mud and the ridged zimmerit paste and so on. Also the tank does not move around so much as a plane, it generally stays upright and only turns slowly so you don't notice the weird shadows and highlights crawling across the top of the wing that you see on a maneuvering aircraft with an overdone normal or bump map.
I find so long as I keep all the effects sharp I don't get much in the way of artefacts on GVs. I did once try to get a rough armour effect by using a layer of black and white dots which were gaussian blurred but this layer gave an odd glazed looking effect when viewed in the game. The same black and white dot layer looked a lot better with sharp dots and no blurring. The main place I get compression artefacts on planes is on the skin height variation effect I use, which is just a blurred copy of the rivet layer. Here the effect has to be blurred and it is just a question of keeping it fairly subtle so the artefacts are only just visible close up but not distracting.
