If it passes (I highly doubt it will in its current form), its not going into effect until 2018, and I think all vehicles prior to 2018 are exempt.
So if im reading it correctly, there will be big issues for deleting your new 2018 Duramax, but model years prior to that, you're just subject to the emissions laws and inspections of your local county....exact same as it is today.
Honestly, by the time 2018 comes around, tuning will be all but dead anyways. The EPA regs are going to be the least of your concerns/roadblocks. GM will be rolling out encrypted GMLAN (FD-CAN) into car/truck ECU's within the next couple years...in order to reflash any module, it needs to be unlocked with a 5-byte seed/key that is only stored in the cloud on GM servers.
5-bytes. Thats over 1-trillion possible key combinations.
The reason for going to an encrypted databus/ECU communications is mostly due to increasing cyberthreats of wirelessly hacking vehicles and taking over control of critical systems while people are driving. And then another benefit obviously being that only GM software can be reflashed, so no more tuning.