“Mage Power” includes itself, Field Spell Cards on your side of the field, and monsters acting like Equip Spell Cards, such as Union Monsters.
If you control “Cannon Soldier” equipped with “Mage Power”, and you activate “Limiter Removal”, then the ATK of “Cannon Soldier” is (1400 + 500 + 500) x 2 = 4800.
Netrep Rulings:
“Mage Power” counts itself toward its effect. (Example: if you Equip this Spell Card to your monster and “Mage Power” is the only Spell Card in your Spell and Trap Card Zone, the equipped monster would increase its ATK and DEF by 500 points.)
A Field Spell Card in your Field Spell Card Zone is included in the effect of your “Mage Power.
It does not matter if the Spell and/or Trap card(s) on your side of the field are face-up or face-down.
The only Spell and Trap cards included in the effect of “Mage Power” are the ones controlled by the person that is in control of the “Mage Power.” If your opponent takes control of your monster that is equipped with your “Mage Power,” “Mage Power” would still only count the Spell and Trap Cards on your side of the field. So if I have a Gemini Elf equipped with “Mage Power” and 2 Set Spell/Trap Cards and my opponent takes control of the equipped “Gemini Elf,” they would not count how many Spell and Trap cards are on their side of the field. Only the ones on my side of the field would be included in the effect of “Mage Power.”