You can always get higher compression pistons via Wiseco. They make custom pistons.
You can also get your crank and cams lightened, balanced, and cryotreated. Same with your rods.
I'm sure you can find some bigger injectors and a faster flowing fuel pump pretty easily.
If our car wasn't drive by wire, I'd say that you can put a bigger TB on it...then again, they might all use the same basic signal and could possibly work....
Reading about methanol injection was a pretty fun read when I had my Reno.
Lighter wheels will make a difference. It won't increase HP/Torque, but you will notice that you can burn out easier and accelerate faster. Lighter brakes do the same effect, but I haven't looked for them or heard of them for the G6 yet.
Decreasing heat in your engine bay will increase power and mileage, too. Make some hood vents, use some "hood shims", and/or heat wrap your exhaust. There's a material out there that people use to make "TB spacers" which don't transfer heat. You could use the same material on your exhaust manifolds and even your intake manifold to knock down the AIT and the temperature in the cylinders. Getting a cooler thermostat might help you out as well. Aluminum radiator and upgraded engine fans would help, too.
There's a ton of universal stuff you can do to any and every car. If you know what you're doing, you don't have to buy parts from anyone because you can just make your own.