I'm a fan of the duratec, though i don't think i'll get around to fitting one. A few minor points:
It's a ford/mazda engine introduced in 2001 (zetec was '94ish?). Stock 2.0 engine produce 150ish bhp, but can release up to 200 with fuelling/exhaust.
The ST170 is a zetec with a fancy head, and like you say, wont release any more power (without aftermarket non-vvt cams)