As I stated the difference in AFR between a 140 and 160 degree thermostat is VERY small for a Merc EFI motor based on the coolant tables I've seen so it doesn't surprise me that it ran ok. Having said that I would recommend the 160 thermostat IF that's whats supposed to be in it.

Glad you got the problem sorted out.

I would not do this on the 496 Volvo s nor the 496 Merc nor the closed cooled marine engines that use a 170 degree T stat.
I actually had a customer do this on a 496 Volvo changed to the lower T stat -- and that ran rich - 1 year old plugs were very poor when I pulled them. - he said his engine was running rich. - I told him I was going to change back to the stock T stat and all good now - one year on these plugs and they look a whole lot better and he said his engine does not smell as rich when running and idling around the docks.