From my understanding anything under 80c isn't a big deal for a long term load. If your main goal is to mine bitcoin for profit, it is for sure more economical to keep the chips cooler. As in, you'll consume less electricity for a given TH. But, my main goal is to heat my house so I need the chips to be warm. When the chips are at 75-80c, I get an outlet air temperature of 50C, and by the time it vents in to my house it's at 35-40C. That keeps a main floor living space at about low 20c when it's 0-10 outside. My point being, I don't want to keep the chips cooler and I'm fairly certain any long term lifespan loss is negligible. Check back in a few years and I'll tell you. Haha.