Can we stay with the omega theme please,
You've still not told us what exactly you want to achieve.
Engine cone filters decrease performance, you get a better sound, but at a big cost.
There are no real exhaust mods, post cat, the system is quite free flowing.
Possible to do exhaust manifolds, but these would have to be custom made, but I think you will have little gain.
You can't chip a 3.2, so nothing to do there.
Replacement diff's with lower ratio like ex-police ones are quicker off the mark, i've driven one, its quite noticeable. But most are shagged by now and you get higher rpm for motorway cruise, so worse mpg. You want low rpms for high speed (which is what the Omega is all about)
A damn good service will increase performance, but if you are serious about wanting more go, you are going to have to think V8.
If you want "performance parts" you need to by a Saxo to mod