[Closed] Triathletes: why don’t they use mtb spd shoes and pedals?

Whilst no shoe sole is perfectly stiff I’m pretty convinced I can’t tell the size of the cleat area through a top end carbon road shoe. The only real different would be that any play in the system felt as ‘wobble’ would be accentuated by a smaller cleat.

I'd agree with this. It's the stiffness of the sole that is important, not the pedal system. Road pedals have a bit less play and feel a bit more "positive" than SPDs, but I suspect the performance differences are negligible if you are using stiff carbon soled shoes on both systems. I've certainly never noticed pressure points when using SPDs.

As for the triathlon transition question, it makes sense that it would be slightly quicker to just run straight to the bike and strap on your shoes while you are rolling, instead of stopping to put on SPD shoes first. I used to do the occasional duathlon, and sitting down to change shoes wastes a surprising amount of time in transition.