I think your just plain wrong. Where you said 'OE' do you mean aftermarket?. The K&N pannel filter improves the air flow and will have a small benefit on perfrmance. Cone filters without a cold air feed will of course hamper performace. An induction kit with a modified cold air intake will be superior to the current set up. The standard airbox is far from optimum. Sucking air from beside the battery? Both the 120 & 180 will benefit from air intake mods.OE filters will not give you any extra as the standard box is already optimum. You could replace with an ITG or K&N panel filter to give yourself some longer life, but you won't get any performance increases, and will more likely see decreases more than anything.
Only the 180 will get anything out of an aftermarket filter, and even then the gains aren't massive.
Welcome To Mg Ownership mate,hi all, got a mg zs 120 2002, had it 5 months love it 1st mg/rover ever had. looking to add some mods, can any1 recommend exhaust filter etc and which 1s works best with this motor, cheers all