For photoperiod, I switched down to 6 hours of MH per day, and have better colors than I did with 10 hours. I didn't have any tissue problems with 10, but colors weren't great.
My temps were always all over the place, never an issue either, so I wouldn't worry about temp at all.
Your issues over the summer likely weren't a direct result of lighting, but reducing full intensity can have benefits, such as reducing photoinhibition.
I also keep my T5 actinic on all day, mostly for color. I could turn it off, just don't have any extra pins for my timer
