A fifteen degree split is a little on the high side. I was running about 75 day to 60 night and noticed a big difference in flower production and ripening time. It was a difference from warmer temps, but after a few extra weeks waiting I managed to fill up a bunch of jars. Bottom line, yea the temps are going to cause problems, but not enough to kill your plants, just enough to make the harvest be not all that it can be.