It comes down to definition. Here is an article from before the 2017 that appears to define superstar much more loosely than I do.
www.mlb.com/news/who-are-mlbs-next-superstars/c-215179722But I have a hard time with a definition of superstar that doesn't include a guy voted into the Hall of Fame by the writers.
Let's suppose we have three tiers of Hall of Famers:
Tier 1 -- The no-doubters, plus arguably guys like Bonds and Clemens. Arguably. The no-doubters usually are voted in almost immediately and are considered among the top players at their position.
Tier 2 -- The guys who are voted in by the writers and with whom most agree.
Tier 3 -- Mostly guys who are voted in by the Veterans Committee or whatever it's called now. Those with strict Hall of Fame requirements -- such as I -- might argue whether some or many of these guys belong.
Barring the unexpected, I think Buster will get into my definition of Tier 2.
There are a decent number of guys who haven't yet met the years requirement but are pretty much in when they do. In this group I would put Buster, Madison (to a slightly lesser extent), Trout, Kershaw, Cano, Beltre, possibly Beltran, Votto, Scherzer, Verlander, Cabrera and Pujols. Some of these guys have the years in but were still playing in 2017.
There are many younger guys who look like they have a fine shot, although clearly a guy with few years in can easily stumble. Tim Lincecum would be a fine example. Pitchers are probably more vulnerable than everyday players.
Here is how Fan Graphs defines players from scrub to superstar:
Scrub -- 0 to 1 WAR
Role Player -- 1 to 2 WAR
Solid Starter -- 2 to 3 WAR
Good Starter -- 3 to 4 WAR
All-Star -- 4 to 5 WAR
Superstar -- 5 to 6 WAR
MVP -- 6+ WAR
Regarding Buster, if we consider his call up season of 2010 and his injury-curtailed 2011 season as one, he has completed seven seasons. One of those was at the MVP level (He won the award.), three were at the Superstar level, and three were at the All-Star level.
I think because of the nature of the position, it's harder to accumulate WAR at catcher. For instance, Johnny Bench played at the MVP level four times; Superstar twice; All-Star six times; Good Player once; Role Player once; and Scrub twice. Bench was really good, yet by this definition, he played at a Superstar level or higher in only six of his call it 16 seasons.
After essentially seven seasons, Buster's WAR is right at half Bench's WAR in 16 seasons. It appears unlikely that Buster will catch Johnny, with the primary difference being fielding.
Madison Bumgarner has been at the All-Star level twice and the Superstar level once. He's still borderline, but at the moment his postseason stardom might well swing it for him.
If we want to see a guy who has played at an MVP level each and every full season, check out Mike Trout. WAR calculated that Trout has played at an MVP level in each of his full seasons, including last season, when he missed over a quarter of the season. Twice WAR has equaled the All-Star level PLUS the MVP level. Trout's WAR in his first six full seasons has been higher than Willie Mays or Mickey Mantle.
Trout is a generational player, just as Kershaw is a generational pitcher. They will likely go down as two of the top 20 of all-time.