Maybe I'm wrong, but I feel like there is a difference between tuning any vehicle with the emissions controls in place, and tuning with a reduction or defeat of those controls.
GDE's tune allowed for disabling DEF injection. Renntech is huge. If they were tuning diesels with a goal of disabling emissions, they likely would have been visited by the EPA by now.
Well, it's complicated...
The CAA prohibits changes not only to physical emissions devices, but all "elements of design", which include software modifications, so generally, ANY modification from the configuration that was originally certified are prohibited.
However, there is also class of configuration called "Adjustable Parameters" which provide some leeway. Here is what the Act says about them:
Engines that have adjustable parameters must meet all the requirements of this part for any adjustment in the physically adjustable range. An operating parameter is not considered adjustable if you permanently seal it or if it is not normally accessible using ordinary tools. We may require that you set adjustable parameters to any specification within the adjustable range during any testing, including certification testing, production-line testing, or in-use testing. You may ask us to limit idle-speed or carburetor adjustments to a smaller range than the physically adjustable range if you show us that the engine will not be adjusted outside of this smaller range during in-use operation without significantly degrading engine performance.
Provides the text of the 40 CFR 1054.115 - What other requirements apply? (CFR).
www.govregs.com
In other words, you CAN alter adjustable parameters, but a parameter only qualifies as such if it has been tested to certify that all available settings of the parameter meet the emissions requirements.
So, you are correct that it is
possible that the Renntech tunes only alter adjustable parameters, in which case they would be legal. This is probably true of the products that they advertise as "50 state legal". But, the Sprinter product is not one of these. Somehow, I suspect that there is a reason for this.
As for Renntec being "huge", this is no evidence that they are within a law. If you browse the list of recent enforcement actions here:
The Clean Air Act requires new engines and equipment sold or distributed in the United States to be certified to meet EPA-established emissions requirements to protect public health and the environment from air pollution.
www.epa.gov
you will find many companies much, much larger than Renntech (Cummings, for just one example).