I dont think this law being signed is going to encourage very many to rush out and get their machinist buddy to make them a can. The problems and dangers seem to be mostly well known. Granted I am sure there might be some who take this as clearance to stick a oil filter on the end of their firearm, write "Made in Tejas" on it and think they are okay...but I doubt many would take that path.
So, perhaps a test case will be in the offing. But I think most people are looking at this as a hopeful gesture, but not trusting it to protect from prosecution. I know I am looking at it that way.