The escort manifold is crap, you'll gain alot of power just by changing that, it gets more restrictive the more power you're running. Area six make one and there are more coming onto the market all the time.

The Zetec block and crack is capable of running massive power, 5-600bhp can be had reliably with the right mods so not much gain in swapping to a YB. One issue is the oil pump which doesn't like to rev too high, a well balanced engine will help with it's reliability but if you want to go past 8000rpm you might want to go dry sump.

If you want 400+bhp you're probably going to want to do some headwork on the inlet side, that'll help produce power without silly amounts of boost.