Probably fine, and you don't need a cooler. For this size of micro the problem isn't heat generation so much as things not settling quickly enough before the next instruction executes. The Maker of the chips has tested them across the full range of temperature and voltage ranges and obviously found that at some extreme combination of values for some percentage of parts the none recommended clock speeds failed.
So at room temperature for a hobby project it's just fine.
If you are building a logging device that had to operate in the great outdoors under battery power then maybe not quite such a good idea.
Other than the Teen 3.6 issues around EEPROM at high speeds not aware of any report problems with people using overclock within the options offered by TeensyDuino. There are some further options buried in the code, and some of them have caused problems with code stability or peripherals not working hence being 'un-comment lines at own risk' things.
