GM used to sell a kit for the turbocharged Cobalt that added 30 HP without voiding the warranty. This was basically just a reflash of the ECM. I don't know if they sell a similar kit for the Caddy.
As for how much power you can really get without caring about reliability, the sky is the limit. GM racing made ~900 HP from the Ecotec for sport-compact drag racing, but you can get away with a lot when you only have to run 9 seconds.
You could raise the engine speed and make more power by putting longer duration cams in it. The stock 2.0L turbo makes peak power at 5500 rpm, but the stock 2.4L NA makes peak power at 6700 rpm. So that would be ~20% increase in power. I estimate you could get maybe 360 HP from the stock engine components before you risk hurting it in a daily driver situation with the occasional track day thrown in. Past that you need better pistons, valves, and head gasket to cope with the heat and pressure.
If you really want to go for high-speed, high-boost operation and get the forged crank, rods, and pistons, 500 HP would be achievable, and the engine may last a season's worth of track driving. Speed costs money, how fast do you want to go?