I asked the same question on a vectra c forum and the reply back was "just after the 4 into 1 exhaust junction, beside the bulkhead.. there is no cat under the car on the 2.2's... to remove the pre-cat will require you to cut open the can and re-weld it .." He's had 2.2 for years but said on the C its a pig to get out and defo not worth all the work. HTH
The Forum in question is Vectra-C.com, best place for Vectra C's on the net, not many Z22SE's on there though so most peeps no nothing about them. I'd say here is still your best place for all things engine but chassis wise give them a look.