Kicking new life into an old thread... your comments about the putting on of nose clips or mask being a distraction, yes I agree. In my pre-performance routine I aim to breath through the nose up until just before 30 seconds before the dive (here I'm talking pool events) with the intention to have the nose clip in place at 30 seconds and then mouth breath until I dive. Much harder to do for static if you favour floating on your back prior to a performance as you have to fiddle about with a nose clip while supporting your face and arms out of the water and that's not a good definition of relaxation.
I believe in the whole nose breathing thing so I bias my prep to enable that and have switched to resting on the pool edge before a static so I have better control over myself and my equipment (nose clip).
The crucial question for me is, if you switch from nasal breathing to mouth breathing, how long after the switch do you lose the benefit of having breathed through your nose at all?