My buddy has twin 502mag mpi's in a 312 Fastech . Last year ran 160-165. Now running 160 cruising but at wot it climbing
to 180 and higher set off alarm on one motor. He already did new impellers, new thermostats. Same thing.
He even checked the inlet off the Bravo and it was clear...
Any thoughts??? I think it's the circulating pumps on the motor. But both going at the same time is odd.

More than likely he has some type of obstruction preventing enough water flow.
The first place I would look is at the entrance to the oil coolers.

Another consideration if water system checks out is it may be running lean. Might what to run an O2 sensor to see what the a/f ratio is running wide open.

I had the same issue with a 454mag mpi. All the normally associated things checked out but temp would still climb under load. I pulled the outdrive off and found a cluster of zebra mussels partially blocking the water pickup tube just inside of the gimbal housing. After removing the mussels everything was back to normal, and I only boat in fresh water,....saltwater guys probably see this all the time

Doesn't sound like he has any water pressure gauges. Add some of those so at least you can see what he has for pressure, and go from there.