The R9 390 is basically the same as the R9 290. By default the R9 290/390 are designed to run up to 94C and then the fan speed will increase to maintain 94C. The fans are limited to 47% by the default driver (for noise reasons), and after the GPU reaches 94C and 47% fan speed, then the GPU clock rate will lower to maintain that 94C temperature. I set my fans to go up to 100% and the temperature to 94C, so my two R9 290's in crossfire always run at 94C and 100% clock speed (when needed). They never slow down during a very taxing game, and the fan speed just goes to what ever speed it needs to maintain 94C.
If your cooling is good enough to keep it below 94C at full load and full clock rate, and the fans aren't too loud, then that is even better.