afaik, imho, tbh... save your money. Be patient and look for proven methods. In the mean time make sure the servicing is up to date, and the bushes are ok front and back. Service your ac, do the belts and filters. Etc.
Then, hopefully, some answers will be ready. Ie where is the weakest link/restriction in the engine? From what i gather the exhaust manifolds are suspected to be most restrictive, hence the advice given by others perhaps. The stock items feed into a plenum rather than use the scavenging effect of a long downpipe which is also more expensive to make than the stock cast items. Why? Cost? Tuned for grunt and a flat torque curve? Is this at the expense of fuel economy for comfort? Or will free er flowing manifolds give better economy but a peaky tune, i cant beleive economy could get worse but thats probable if it flows more air. You get the general drift.
Once the above is known, then it may be worth further improvements, for instance there seems little point fitting larger throttle bodies and bigger bore straight through exhaust if the manifolds tie a knot in the system in between.
You may get some gains here and there, but at what cost? More noise in the cabin? Worse economy? Loss of low down grunt where you need it most?
I find it amazing that a small tuning firm can turn over millions of pounds of r and d on what is an executive car, dont forget, with a panel filter, pipe and a chip without some down sides... Dont you?
The omega is not really designed with world touring cars in mind, and i beleive those that do race it bin the v6 for chevy v8! And a thing of beuty it is too.
Its your car mate, do as you please, personally i love a tinker, but it takes more than a tinker to beat what GM designed.
God im getting old. Nyear