Yes, I am concerned with the low threshold; 20% seems low. If you are talking about a party with 12 people in it, you have three people who are able to cause a problem for the leader. You might have three people who do it for entirely the right reasons, but you might also have, in any mathematical situation, something that's inspired more by politics than by anything else.
It seems that it would be up to the full caucus. It would really depend on what would happen in the full caucus vote then. If you had, say, a politically inspired or opportunistic request for a leadership review and then the full caucus swatted it down, that would be the end of it.
One of my concerns is that we don't know how much time would go by between the leadership review petition and then the vote. If someone were campaigning for leadership, do we give them a month to campaign? Do we give them a week to campaign? It seems to me that would make a huge difference in the evolution of what happened in the party.