This is an interesting proposal.
If you are not aware, the $200 tax stamp payment does not go towards ATF funding.
That money goes into the US Gov't "General" funds.
So, dropping the tax stamp would have no impact on the ATF operating budget or manpower.
However, if they are trying to make the argument that NFA firearms are protected by the 2nd amendment, that is already case law and they are not.
Good question on suppressors as they are not firearms.
Not many years ago there was a lot of support in the US Congress to remove suppressors from the NFA and there was a "hearing protection act" that was to be voted on.
That was never put to a vote because of a mass shooting event.

Richard