Heatsink with forced ventilation always matters, even on a miniature scale. Note that the biggest ICs are the ones which more heats and you need to take care about them; The raspbery distro has a specific line command that take the core junction temperature whch is an valuable information to be monitored. Although this board was not primarily designed to work outdoor, the RPI have a good tolerance to high temperatures. I put one RPI3 with heatsink and a small fan blowing from far, inside a chamber with controled temperature of 70 degrees for almost half hour before it stop to work. I have also hearded about people who made even more aggressive tests, but I didn't have the courage to get over it.