I seem to remember reading in runners world that anything over 60 degrees and you should slow down by 2.5% for every 5 degrees until it gets ridiculously hot.
This calculator
http://www.runworks.com/calculator.html is based on Jack Daniels' book and seems a little more optimistic.
One thing that none of these calculators factor in is the humidity which is just as bad (if not worse) than the heat.
Ned, according to the calculator you should have been able to run 8:20s which is insane if you ask me.
Galloway is a little more realistic with his charts:
Estimated temperature at finish...Slower than goal pace
55-60 degrees.....1%
60-65 degrees.....3%
65-70 degrees.....5%
70-75 degrees.....7%
75-80 degrees.....12%
80-85 degrees.....20%
above 85.....forget it, run for fun
According to Galloway, you should be in the 9:30 range
Its all really based on how you are feeling in my opinion.
Unless it's Florida hot, I don't think the heat will really affect you in a 5K or 10K but I wouldn't want to be running a marathon with temps in the 70s
84 degrees is waaaaay too hot to be doing long runs. Didn't they cancel the Chicago Marathon when it got into the 80s?