Do you have any idea what your humidity level is like at 74? I bought one of those 3M thermostats and they come with a pre-defined setting.. it calls for 78,85 and 82(lat at night). I dont use it.
First year I ran 78 during the 3PM-6AM time. Came on at 3PM so that by 5 it could hit target. Downstairs was fixed at 78. All other times it was set at 85
Last year I ran 76 during the 3PM-AM time. Downstairs fixed at 76. All other times it was set at 82
I saw a very minimal increase in costs last year. This is in FL.
This year I am not sure what it will be, I have since added a return and balanced the upstairs, so I believe the the upstairs could go back to 78 and be comfortable.
Do you have progress energy? They have a cool little feature on the website where you can see your bills and the highs/low for the month. This way you can evaluate your changes with reference to outdoor temp/humidity.
Also another point of reference as you mentioned- insulation. Upper ceiling temps will hit 90 in the smack of summer for us. Attic temps I have recorded 125.