The '170 is pretty torquey for a N/A 2.0 and makes it across the revs, I've no idea what effect the manifold and exhaust has on a normal zetec but it can't be that spectactular surely?!

The ST170 does indeed have better suspension and brakes, as well as the Getrag 'box.... Imo if you can afford a decent ST170 (they're now pretty affordable!) it doesn't make sense to start trying to make one out if another model.