What would be the best and most reliable computer programmer for a 99 V6 Chevy Camaro?

The closest thing I can think of that will do what you want is an AEM engine management system. It connects between your stock ECM and your wiring harness to allow you to override sensors and controls. Performance enthusiasts use them with things like turbo setups, nitrous oxide, and other engine modifications. You can get a little power (or economy) out of one without other mods – but you won’t like the price tag.

The best thing you could do for your engine is a tune-up. Plugs, wires, valve clearance, fuel pressure, maybe a K&N filter. That would help with both power and economy, and won’t cost more than a couple of hundred bucks.