You should be good to go with that size. I have a 50qt. I preheat the tun with all my mash water that is around 15 degrees higher than my calculated strike temperature. For example, if I'd like to target a 155F mash temperature and my calculation states I need 165F strike water, I'll heat it up to 180F and throw it all into my cooler and close the lid. I'll wait until the temperature reaches 165F and then throw in my crushed grain.
I've had issues reaching my target temperature on lower gravity beers (ie, less grain, 5-8 lbs) even with adding more water. I attribute that to the amount of headspace above the grain rather than the material of my cooler. I may lose up to 5 degrees during the hour long mash. I haven't yet tried reducing the headspace by putting some insulation on top of the grain bed or by wrapping the cooler with a blanket. In the end though, my beer has not disappointed me, so the temperature control hasn't been a batch-dumper issue for me. Good luck!