hardneed steel against aluminum.. Al will loose!

If you ream out your back plate with a reamer, I doubt you will ruin it. Gum it up maybe, but not ruin it.. Use a bit of 'cutting fluid' while you 'ream', Use a squirt of WD-40

And yes, you can buy metric drill bits..
8mm = 0.314" or 5/16" (0.3125)
10mm = .3937 or 25/64" (0.390625)