for whether mohammed ali was a NOI member, I think he was but then they kicked him out for some reason and he stopped believing in their form of islam.
I have never met an NOI member (seen one guy on a street corner preaching wearing a bowtie while holding a newspaper in Oakland though). I've seen a lot of normal muslims though, I doubt they sympathize with the NOI beliefs because they usually marry outside of their race and just don't seem insane.
Is NOI seriously big enough to consider a religion (instead of a cult) today?
No, Ali and pretty much anyone close to Malcolm left after he was assassinated. After Malcolm was murdered he looked out orthodox Islam.
NOI now accepts white members and stuff like that. It's very very niche. Elijah Muhammod's son Warith was made leader when he died, and he pretty much killed off NOI because he didn't believe in their pro-black principles and formed an orthodox Sunni Muslim organization. In the 1980's Farrakhan, a member of the NOI revived it. It is not less extreme but it still serves the black community.
In some ways they act as patrol. The neighborhood buys them out, and the NOI literally cleans up the block. Drug dealers are scared of them and don't mess with them. They decide to take their dealing somewhere else.
While the NOI is extremist in nature, and assassinated one of my heroes, I still respect them for what they have done for the black community and they pretty much prove that it is possible for the people of my race to come together and prove they're not nappy headed distinguished black fellows.