[Cockerill]

When you say there's .5mm of play, is that along the the shaft axis, or in/out of mesh?

The only time I've seen play that allows the mesh to change is when there has been a dodgy bearing or a bearing has seized at some point, causing the casing to warp around a bearing hole.

The steel gears will allow give a greater safety margin for any play, but the alloy housings certainly aren't needed. I'm still yet to put them on my car, because the plastic ones are working fine for me. I also run the Standard Schumacher rubber sealed bearings.