I would say closer to 10-20 deg in the city, at least at the moment...it'll stay warmer in the city because the heat is retained in the buildings and asphalt. The bigger drops are more common in the middle of the desert. For example, where I live is usually 5-10 degrees cooler than central Phoenix because there's more desert.

I'm still commuting and usually leave work around 5:30-6:00pm and I manage just fine although it is more pleasant to ride in the morning.

The biggest issue with the heat is that I feel it zapping my energy - even when staying hydrated. I tried riding last week at 3:30 in the afternoon, I was drinking a ton of water but I just didn't have the urge to continue after an hour and went back home.

The other issue in the summer is the monsoons, which bring higher humidity - still not a lot, but when its 110 outside you can feel and see the humidity. The air just feels thick.