The SVT has a "DSI" manifold. It uses 2 different sets of runners..longer at low rpms and shorter at higher rpms. Bigger isn't quite the right word, but yes, it is better and will bolt to a zetec, but will require some method of activation for the runners. (rpm switch, etc.)
What your refering to with "decking" is actually milling. Shaving material to increase compression. You can do this, it will work...i've seen people mill as much as .060. Do not reccomend this on a stock bottom end though.
Dunno anyone that has bored out a zetec really. Tom at Focus-Power has a stoker kit now though
(2.3L)
SVT does have stronger internals, but the increased compression is from the different head I believe (which has VCT to btw).
If your looking for real power from a Zetec, I strongly reccommend an aftermarket head, as the stock one flows like crap.