Since fortification is under combat powers, if I pick power synergy which gives 20% power damage, will it buff ammo powers like incendiary or disruptor?
It does! You can easily verify this by checking your ammo power in the Group Menu with and without fortification activated. E.G. Incendiary ammo Rank 1 is 10% Dmg Activate Fortification Rank 5 Choice 2 Check back to the Group Menu. Incendiary Ammo is 13% DMG
But understand: Buff percentages doesn't sum up. 30% and 40% (eg from Combat Mastery) aren't 70% more damage! You may calculate from the so called increased basic value. In simple terms: Ammo 10% DMG x 40% Buff Combat Mastery = 14% DMG x 30% Buff Fortification = 18% DMG
Nonetheless. If you choose all Ammo Dmg Upgrades and the Ammo Uprgrade in CM, you reach 45% DMG with IA. Activate Fortification and you should have 59%. But alas! There are only 54%. Can't explain this, But 9% more Dmg is better then nomore DMG. ;-)