from sdsefi.com FAQ/Tech

Why will an "off the shelf" ECU Disappoint?

The first reason is that on most naturally aspirated engines operating
on pump fuel, the only way to achieve tangible power gains is by
increasing airflow through the engine. Chips cannot do this therefore they cannot make much difference in power output. Chip re-programmers can richen the mixture slightly at full throttle and advance the ignition timing slightly perhaps but this would be at the expense of lowering the factory safety factors for detonation and emissions. The absolute maximum gain in this instance would be on the order of 5% and could be as little as 0%. Most independent tests that I have seen on performance chips for naturally aspirated engines have indeed shown minimal or no gains in acceleration.

Some were slower than the factory chip.

Chips for use in factory stock turbocharged applications can increase
power substantially in some cases by raising the boost pressure. This
again reduces the factory detonation limits and you risk engine damage.
Without increasing fuel octane, you are asking for trouble especially if your engine does not have a knock sensor.