You're probably right, the rocker panels on the lamborghini miura are painted silver.
However, at the time that I was still conceptualizing the project I wanted the trim to be shiny and there'd be no way that I could get silver paint to shine like polished aluminum. Then by the time the trim panels had been fabricated, giving them a high polish called attention to some of the imperfections on 'em. Hence, I opted to just let them tarnish a bit.
Again you're right about the inspiration! Here's a model of a Lotus Europa John Player Special Edition the appearance of which I try to emulate:
I thought about that. There are a few guys here with side skirts on their cars and they've had them for quite some time - they don't seem like they have any accelerated rust issues. But then again, they have yet to take down those skirts to inspect the sills. And also, I think that rust forms from the inside of the sills rather than from the outside, moreover, the outside is heavily coated with the stone guard. Thus, the rocker panel covers are effectively resting on that thick layer of stone guard and not on the metal. If there's any rust that's going to form, it's going to be from the other side of the metal, or in between the stone guard and the sill. Hence, the trim panel shouldn't have contributed nor have anything to do with that rust formation. If any rust formation should occur it would probably form at the drill hole, of which there's only one on each side - no more than that which is required for side skirts.
Yes, that accelerated rusting might occur does bother me, but that only means that I should be a bit more vigilant in looking out for it.
Side skirts are fiberglass. Your aluminium sill covers will likely to rub on the sill paint, and go through it. Be repared for electrolytic corrosion; like when the sill plates rub through. Remember, the Miata/MX5/Roadster body twists a lot.
Might be fine; but I would install them so you can easily take them off for, say, an annual inspection and clean. Might be worth fitting adhesive pads to space the cover off the sills.