id put my money on the 57i being the problem. zetecs tend to run a bit warm and sometimes have issues with torque loss when induction kits are fitted. a simple but effective way around this and to make the car feel so much better is to source a flexible intake pipe (maybe 1.5-2 feet long and suitably sized) and connect that to the MAF then rout the other end down behind the bumper where the lower bits of the airbox used to be and mount the cone filter to that and secure it to a bracket or something.

ive experimented with panel filters and modded airboxes, induction kits, heat shields, etc. and this is the best solution. you should notice an improvement instantly and if mounted low enough produced quite a lairy sound when you step on it compared to the sound of an induction kit or panel filter.

take into consideration the new location of the filter when the road is wet and try to avoid splashing through puddles on that side unless you want to make a guard for the filter, also remember to clean the filter regularly as it is in a somewhat open location. out of all the filters ive tried i found stainless pleated mesh filters work the best as they can be cleaned with a brush and soaked in degreaser to completely clean them and then oiled up with a little crafty spraying of some wd-40 or gt-85.

once you try this setup you will never go back to a plain old induction kit or panel filter again.