One player winning one tournament with one master doesn't automatically too strong, but if the tournament is high profile it is an indicator, because the player basically played with a handicap.
3 players being in the top half of a tournament is usually meaningless information.
In games that are very skill intensive, if a list is too strong and popular, there are usually a lot of people that do well with it and a lot of people that aren't, because the worst players gravitate to lists that are pereceived to be the best. This is why % of the field and win rates across the board aren't the best tools to rate popular lists.
Having one master being a large part of the meta and winning tournaments after tournaments usually signifies balance problems, because the meta can't adjust to the master.
Balance should be always determined by the current GG. If Tara needs Corrupted Idols to be good, then she should be buffed if it isn't and nerfed if it is. Otherwise she is a balance problem in and on itself.
From a faction balance perspecitive the 8,8,8,0 would most likely be broken. Even if there isn't a master that completely shore up the weakness (for example x/x/x/4, it would still be really bad designed.
If I only play 1 master it depends on what my skill level, goal and the meta is. If I want to win the tournament and am not ahead of the competition, I take the list that gives me the most polarized results in the field. So for example if most people are taking lists that are extremly good at the 4th scheme, taking the 8,8,8,0 would probably be the better choice, even if I lose the one scheme 100% of the time. If I am confident that I will win against players even if I play a 6 list and my oponent an 8, then the 6,6,6,6 is the better choice.
So in short, better players usualls gravitate towards "balanced" lists, while worse towards polarised ones to have a chance against players that play better than them.
The overall rating is only important if you don't play another master that shores up the weakness. For the faction balance the 7,7,7,1 is meaningless if there is a 8,8,8,0 and a 6,6,6,6 in faction.
Balance should always be determined in relation to the current GG and balance changes should be made accordingly.
While this would be good if you want to balance the masters, faction balance most likely would suffer through such a design approach.