As mentioned, you can get a custom chip from Sinister Performance
(gmtuners). This, however, is no off the shelf item. You need a good scantool to record data to send it to them.
I would suggest a tune up (plugs, wires, acdelco O2, etc) before you get the scantool data. This will give you better data and therefore a better tune.
I had a few chips burned by them and they will not stop burning chips till your satisfied. Very stand up company.
If you plan on modding your car more, the chip should come later. BUT a scantool should come first.