For starters a 65mm has been dyno'd to drop power on even moderately modified SVTs.
It sounds like:
The throttle plate is not closing. (does it fully close?)
They didn't plug the bypass port.
They touched any part of the inner TB where the plate rests.
They shaved the plate down too much or knife edged both sides.
Air is bypassing the top of the gasket were the 65mm flange does not cover.
There's a quick list. I am sure to have missed something.
However from the other things you mentioned it really sounds like whoever did this doesn't/didn't know what changes must be made to the 65mm to get it to work on the Duratec. So going from that who knows what else they did not do???
