The battery tray will only fall off if you don't do the screws right. Looking also at the piece itself, I could see where the small tabs on the tray could brake off. But only if the person is being careless.
Pre prduction cars. I recall where Masami had some pulleys made for his Dogfighter/Works car at one Worlds.. They were aluminum also. Production parts were nylon/plastic. So I wouldn't cash in anything yet on diff materials or trans weight. Also sometimes accuracy, smoothness, and balance can greatly offset weight. especially when you may be talking an ounce, if that, of difference..
Aren't tracks also rougher in England compaired to the US? You want an indestructable car? Don't get a high speed racer. Get a 9.6 V Tyco.