It's not emissions that is the problem with adding performance today. In the dark days of emissions it was emissions at the cost of performance and efficiency. Then it become emissions and fuel efficiency at the cost of performance. It took a long time before engineers started figuring out how to have all 3; Good emissions, good efficiency, good performance. The reason it's so hard to get meaningful gains without a lot of work on a modern engine is because manufacturers are doing such a good job of getting everything they can out of these powertrains. As such, simple bolt-ons don't add much, if anything, anymore. If the manufacturers can incorporate all of these things themselves to get max performance out of the box there's just not much left on the table for the aftermarket. Nothing to do with emissions limiting performance. Even cats aren't a restriction anymore.Most dyno results from the better established companies are accurate. Like KNN they are pretty spot on but if you look they are not big numbers either.
It used to be to make power it was intake, Carb, Headers and cam to make power. Today with emissions that is not as cheap or easy anymore. Everyone today wants bolt on performance in a box.
But that really means a good set of heads and a supercharger or Turbo system.
The government does watch this and companies like KNN are pretty much honest on these numbers and they will even not list applications of any model they have not tested the vehicle for power or fit.
The flip side is mfrs also have to keep refinement, driveability, and serviceability in mind as well.
As far as the claims made by companies like K&N, they avoid legal issues by stating "up to" a certain gain. They don't guarantee anything. If you gained 0 you can't claim fraid because they never claimed you'd see any gain whatsoever, only that you might see "up to" a certain gain. That is why the dyno charts really don't matter and shouldn't be trusted, they don't have to be accurate because they aren't making any guarantees.