Quoted:
Here's the thing, you need to view all claims by the tuner with a grain of salt. A car's engine is little more than an explosion machine that works on the principle of chemically attaining the most efficient explosion through the right ratio of air to fuel and mechanically attaining the most/best explosions by flowing air through the engine as efficiently as possible. Anything that a tuner does must necessarily perfect the air:fuel ratio and/or maintain an ideal air:fuel ratio while in light of a greater flow of air than the manufacturer anticipated and/or change when the spark plugs spark. That's it.

So, if someone says that a tune alone is going to net 50 horsepower on a naturally aspirated engine with no mechanical changes, you should immediately wonder how that's possible. In what way did the manufacturer tune the car so imperfectly that they left that much horsepower on the table? It's basically an absurd proposition on a modern vehicle.

Now, if you change mechanical components that increase airflow through the engine, then, sure, suddenly a factory tune may be inadequate, and an aftermarket tune will adjust the amount of fuel injected to work in conjunction with this new-found airflow, and you make a bunch of extra horsepower.

Further, if your car is turboed, many factory ECU's now control the amount of boost the turbo puts out, so a simple tune can make considerably more power precisely because you are increasing airflow through the engine so as to take advantage of the fuel/spark magic of the new tune.

A couple of examples: My naturally aspirated Corvette Z06 gained basically no horse power from a tune alone. But the addition of intake and exhaust plus a tune gained 40 whp. My Turboed Cadilac ATS-V, however, has its turbos regulated by the ECU, and a simple tune will net a 100 whp gain BECAUSE THE TUNE IS ALSO EFFECTIVELY MAKING A MECHANICAL CHANGE THAT RESULTS IN MORE AIRFLOW.