I am installing a 2.0 zetec with bike carbs and megajolt, I was going to fit the standard zetec exhaust manifold as it fits nicely, How restrictive is this and how much power would i lose?

There were several different standard exhausts.

The tubular one which loops upwards then down isn't too bad, and its quite easy to cut the flange off leaving a short pipe and weld yout own system on, which is what I did...

I'd forget cast ones..
Rescued attachment myexhaust.jpg

We're using a cast one, but don't have anything to compare it against.
I can tell you it's bleedin' heavy though (and cheap!)

I've got one of THESE! on my spare engine, I intend to weld a turbo flange onto it when I have time and money to build aturbo engine

This is a subject I am very interested in. I have an ST170 manifold, but I keep looking at it and really don't think it will fit in my zetec spitfire
I suspect the tubular std focus one (as on ebay) may be quite good. After all, Ford is not a budget company, and their research dept has a fair old budget. saying that, is there any power difference between the modeo with the cast manifold and the focus with the tubular, or was there no crossover (or even do they use a different manifold when on the engine dyno??)
I half expect the focus to be as good as many aftermarket ones, and I doubt anybody has dyno figures comparing manifolds on the same engine.......of course I have no hard evidence either, but I am cynical about many claims of power gains.