Doubt it very much. 87 octane is no 'dirtier' (whatever that means) than higher octane. The higher the octane the more resistant a fuel is to detonation. High compression engines can cause the fuel to detonate before the spark. Using higher octane in these engines allows the fuel to not detonate until the spark causes it. Using high octane fuel in an engine that does not require it does nothing except make you spend more on gas. So save the money.

If you're finding a difference in when EV mode kicks in I'm guessing it's for some other reason.