I don't think a turbo on only two of the cylinders would work. Also, the zetec can handle more than 3-5lbs of boost. More like 9-10 with only minimal fuel system changes. It can handle more with a built engine and more fuel management,etc.
A properly setup nitrous system (with a 50hp shot or more) with a flip switch would be much more effective (performance and cost wise) as opposed to a ghetto rigged two cylinder turbo.
