Merv, not the valve failure needs to be determined as to how it failed. Valves generally fail due to poor machining of the head/valve seat and the guide. Valve guide wear through rocker geometry or through poor fit and then cargo or lead oxybromine ingress, then sloppy guides mean a rattly valve and then the head breaks off.
To make matters accelerate the CHT will not help if it is very high. There is no doubt the shorter latency of mogas Vs Avgas will have an impact but I doubt it has a serious impact if the cooling airflow is sufficient, but it does not help. There is a big difference from Avgas at about 110RON and 98RON that may well be a bit lower.
Getting back to the rapidly rising CHT, I am not saying it could not ever happen but a rapidly rising CHT is indicitive of a preignition event and not a valve failure.
Valve failure is not normally associated with a rapidly rising CHT. It can often be seen coming with the EGT races in the EMS. But even there not always. We had a C340 owner in a class once with a failed valve and no warning. That is a TSIO 520 Conti.
So was this failure a preignition event or not?
I wonder what the oil change interval was?
I think the valve geometry could be better, the guide material better. Are they post reamed and nice fit?
So many variables and the same is said for the maintenance of them. Don't get me wrong, my IO540 is my preference but many a jab engine does not get the life it should from external factors. But on the other side of the coin the cir clip events showed how easy things come undone too.