6mm poly will protect you against all the axes out there, never had anything go through it. Little Hitter will make nice marks in it but that's about it. Cobalt put a hole in mine back in 2012 but considering that was a hydraulic crusher, it's not surprising.

Used 5mm as a baseplate on Onyx and it worked well, although it eventually developed large crack in it, but that originated from one of the countersunk holes I'd drilled, so it wasn't entirely unexpected.