I know it will retard the timing when its too hot, losing power and making it run hot. However, what I have found is in the winter even a thermostat only 6-12 months old is not strong enough to run at a decent temp for normal driving. Last winter I saw temps of 65 degrees at times when it was cold when commuting. That just seems a waste to me and if a 90 degree stat from Renault is just as sh*t it will run cooler than it should.
It depends how you drive. Driving hard a 90 degree stat makes no sense. Even in normal temps (10-15 degrees - you know you're British when over 15 degrees is hot!) it doesnt make sense with our engine but in winter and for 90% commuting, I can see the relevance, even though Renault fitted a 75 degree stat.
I plan to try a 90 degree stat, just to try over winter and Ill monitor temps. My car is currently 90% commuting and 10% fun.