With respect to 'Combined Arms' Post above, it seems that if a FO gets SOME benefits of being in command (stealth and morale, in his example), he would get ALL of the benefits (i.e. enhanced firepower accuracy) if the HQ has those qualities.
I once had a veteran mortar unit doing an indirect fire mission. The unit was in command of an HQ with a +2 firepower (lightning bolt) rating. The mortar, on two consecutive turns, scored a direct hit on, and destroyed, a halftrack at 600 meters, and followed it up by scoring a direct hit on a Sherman and destroying it at a range of 450 meters. It was game, set, and match.
I believe that your artillery FO WOULD get the benefits of an HQ's firepower rating. BTS, would you care to comment?