I also suggest adding scratches to the diffuse. But i guess if you wanna keep the wear and tear down you could add a buildup of dirt in areas, and then translate that into your spec making those areas more matte to give some variation. I agree with SupRore, if you just lighten the base colour some more it will make details…
There are a lot of really heavy gradients in the normal map, might not look good with a realtime shader. Your texturing looks pretty good, but I would bake a bit of ambient lighting (use a skylight + light tracer and a ground plane) and add a bit more grunge to the diffuse to ensure that it looks good at all angles.