The flex in the glass should still be the same so good bracing is really important.
I made a tank a month or so back that was 900h x 600w x 290d. Was to go in a wall hence the random size. I used 12mm toughened with polished edges, at each end the was a 24mm contact/join. A strip of 12mm polished about 50mm wide top to bottom.
No flex at all.
With not looking at figures and going on what i have done in the past, i would look at 10mm to be the min thickness you would want even if as you said the workings say you could get away with thinner.