Yes the manifold will fit straight on. You need to get the ST catalytic convertor though if you want to run a cat as it runs underneath then up the back of the engine, instead of down the front like the zetec. Most folks just stick a de cat pipe in and worry about MOT emissions later.

As for power gains I'm afraid I couldnt really give you a solid answer. All I know is that the design is much more condusive to flowing gas, being tubular and y branch, but the length of the primary pipes and secondary pipes are almost certainly arranged to benefit the harmonics set up on the VVT 'Duratec' ST engine. Also without figures for flow rates from both heads it would be near impossible to suggest how the effect on gas velocity, from the increase in pipe diameter, would effect performance of the zetec.

In my opinion, as these are both of 2.0 litre displacement I would suggest there are definite gains to be had from fitting the ST manifold purely on the basis of the free flowing design. A friend of mine fitted one to his 2.0 ESP and he reports that the car does seem more responsive.

Hope this helps.