generally they are not a problem as long as they are set so they don't slip but are still free to spin (otherwise you may as well use a spool!!!). Make sure you use heaps of lube becuase that will also keep the temps down.
As said above the kit instructions are pretty good but they are not always 100% and you have to make sure you do retighten the diff after a couple of times on the initial try.
I usually check that my diff isn't slipping about once a meeting anyway as it takes power away.
Mike