We just got back from DL. We were there Fri and Sat. It was really hot (in the 100s) but I have to say it wasn't as bad as I was expecting. You never notice just how many of the queues are air conditioned until you go there when it's so hot. We intentionally rode everything with an outdoor queue early (buzz lightyear, thunder mountain, pirates, haunted mansion). We fastpassed space mountain and splash every time so that wasn't an issue. Indiana is all shaded and partially air conditioned. Star Tours was actually too cold inside; my wife was wishing she had her sweater. The hot day made for a beautiful night... low 80s with a breeze.
If it's 95+ degrees when you go I say just plan to get there when it opens, do whatever you can till 1pm, leave and rest in your hotel till 5pm and come back. That's what we did and we still got on 20 rides (RideMax was quite helpful by the way) and staked out a spot for the fireworks. All the shops were really well air conditioned too so really there's a lot of opportunity to escape the heat... in DL that is... DCA is another story.
You never really appreciate all the shade and a/c in DL until you go to DCA in 100 degrees. That place is seriously horrible in that kind of heat. Very little shade.