Most of YRT's Orion Vs and all of Viva's bus fleet have windows sealed shut. I have ridden plenty of sealed-window buses over the years, and the interior environment is comfortable all year round. The A/C is more than capable of keeping the interior nice and cool.
I find that the sealed-window buses are more comfortable in summer, because otherwise (as many others have pointed out), people open the windows, even when the AC is on.
The downside is that in on hot days, when the A/C fails, the bus needs to be taken out of service. I worry that the TTC may not do this, and it would result in rather uncomfortable streetcar rides.