[aerohead]

As a native Californian, I experienced 'snap' roadside vehicle inspections by the California Highway Patrol. All traffic would be shunted through the inspection. They were in possession of full documentation for everything under the hood, and for any vehicle registered in the state.
If they found 'ANY' modifications, they issued a citation which could be cleared within 30-days, if a certified auto technician signed off on a certificate declaring that all OEM technology had been restored to the vehicle.
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Since most cars are financed, are not 'paid' for, arbitrarily modifying the engine can not only 'void' the manufacturer's warranty, modifications can potentially do irreparable harm to the engine, leaving the 'mortgage holder' on the car with full responsibility to absorb all costs in restoring function. If they expect to operate the car on public roads.
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Any location in the United States which is out of compliance with Clean Air Act air quality standards, is responsible for annual emissions testing, which will mimic EPA certification dynamometer loading while under test. If the modification alters OEM engine 'mapping', the car is liable to fail the inspection.
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Where I live, for 24-years, the car will be required to experience annual emissions testing. If it 'fails', the owner is required to pay up to $600 /year, attempting to bring the car into compliance. Every year for 24-years.
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While a certain modification 'could' improve some narrowly-defined performance parameter, it might simultaneously move the engine out of optimum performance in other arenas of operation.
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Automakers 'designed' their engine and powertrain. They have the most intimate knowledge of their product. They've tested it under every conceivable situation it's likely to ever encounter. It may not be 'ideal', but 'ideal' doesn't exist and never will.
--------------------------------------------------------------------------------------I spent 20-weeks in a university engine test cell. We had full instrumentation, real-time meteorological monitoring, and real-time data from the linked General Electric dynamometer. Everything was adjusted to SAE standards and protocols, and adjusted for ambient test conditions.
--------------------------------------------------------------------------------------Without such stringent controls on testing, I've no idea how an individual could ever have any confidence discerning actual changes to an automobiles performance, given one modification or another, on the possible scale of the mods under discussion.