Any car whose stock engine controls are programmed so poorly that there is room for that kind of increase would never be certified on emissions grounds for sale in the US, EC, Australia, etc., and would produce such poor fuel economy and performance that no one would buy it even if offered.

The engineers who design cars aren't that stupid. If there were a way to get such an edge everyone would have done it years ago - electronic engine controls now have a 40 year history, during which lots and lots of experiments were done.

Don't waste your money.