To test if it's tempered glass wear polarized sunglasses, set your LCD monitor to white, then light the glass from behind. If you see crazy colors and geometric shapes, then it's tempered.
I messed with HOB overflows for 2yrs with my previous reef setup, finally gave up and drilled my tank. Never looked back. When I was setting up my 200g FO, I went directly for 2x1.5inch overflow holes and 3x1inch return holes. Will never consider HOB overflows again.