During MTB XC races, I always get in the aero position for any paved or gravel sections on course. You save many watts and gain free speed by getting low and putting your hands close together when it is safe to do so.
I practice varied cadences in training. For instance, this Wednesday I did 3 sets of 10 x 40/20secs all out. The first 2 sets I was using a cadence in the low 90’s. For the last set I used a cadence in the low 80’s. My average watts was higher for the last set at lower cadence.