Hmmm...you seem to know what you're up against at that time of the year. In my experience, no-see-ums are at their worst mid-May to mid-June. They seem to be at their worst, however, first thing in the morning and again at sundown so they ARE manageable. Temps near 100 are not uncommon during that time, but you could just as well see 80's. Daily water consumption for me increases significantly when temps climb above 90 degrees and I'm hiking 15 to 20 miles a day...close to 3 gallons and I still can't seem to keep my urine clear!
However...if you are mainly doing slot canyons or deep canyons with water(thinking escalante/capitol reef/cedar mesa here) then mid-May to mid-June is perfect! Warm temps and it's before monsoon season, so less chances of a flash flood running into you.