You can mess about with all combinations of 3.2 / 3.0 engines, 2.5/2.6 heads, G cams, 3.2 inlets, etc.... yes, it's true you can get some gains from it, but in reality they will all disappoint you.
The Omega is a heavy, boxy car, it will never (certainly by today's standards) win any notable races, and most modern cars of a similar spec will blow it out of the water.
15 years ago it may have had some benefit, but these days unless you're talking serious work, I think these kinds of upgrades to try and get a few horses out of a heavy Omega barge, are basically like pissing in the wind.
The 2.2 is acceptable, and any V6 is a lovely, comfortable smooth cruiser. That's where it ends for me these days. If you want something "quick", then look away from the Omega, unless you have a significant budget for V8 conversion, etc. (And even then, you have to really love it, because there are much more capable cars out there)......
I will always have a huge soft spot for Omegas, but, I would never waste time trying to tune one. If anything, I'd want to keep it standard / original, and focus on what's important, eg, keeping it rust free, etc.