Well, I bought an new AMG vehicle 3 months ago, so I happen to know that the only way to remove the limiter is to attend one of the AMG training courses. I imagine if you greased enough elbows you would get the limiter removed without doing the course, but there's certainly nothing you can do to order one from factory without the limiter.

And yes, I've done a bit of digging at it appears you are correct - it's not actually the law, just an agreement between manufacturers.

Edit: Ok, I've looked through technical specs of all AMG vehicles, and it looks like every AMG model from A to S class is limited to 155mph unless you do the course, but the AMG GT isn't. So....yeah, I'm just completely wrong on this. My apologies.