You probably don't even need to pay for it. These vehicles are modified; not factory delivered. They probably anticipate/expect the owner to have to reflash the ECU, and include a hand held tuner for exactly this reason.

I'm not familiar with Hennessey products, but this is how Ford Performance handles their "factory" supercharger kits. The tuner isn't included with the kit, but owners expected to have one.

I think you misunderstand how this works. When you get a custom vehicle you don’t really give a shit about regulations or reasons for them. As long as you can register it and renew your registration you will most likely do whatever you want to it. I am fairly familiar with custom motorcycles in the US. Most states and cities have regulations regarding noise levels. The EPA requires bikes to have catalytic converters to reduce emissions. But a bike in the hands of someone who cares about its performance will not stay stock of long. Usually the catalytic converter is removed (it restricts the exhaust system and you can get a few ponies by doing this), and the entire exhaust system is replaced with a shorter and wider one that will let the engine breathe better. That makes it a lot louder. You will also modify the fuel system to run less lean (lean means higher than stoichiometric air/fuel ratio). Lean fueling runs hotter since gasoline doesn’t cool off the cylinders and it doesn’t give you optimal power, but it burns cleaner. Instead you bring the ratio closer to 14:1 to get better power and a cooler running engine.

Most cops won’t pull you over for an exhaust that is too loud unless you keep blasting it in your neighborhood enough for neighbors to complain. And none of them will ever check your fueling system. Manufactures of these devices sell them as “non-public use only”, knowing full well you won’t race your 800 lb Harley. The saving grace is that most people don’t do this. Only a few people buy motorcycles (or high performance cars) and of those only a few modify them to this extent.