You could guess at how many runs they would score with the Runs Created formula. It works pretty well for actual teams.
Not everyone on an All-Star team is the best hitter at that position, since defense helps you earn a spot too, but here was last year’s AL all star lineup:
George Springer, Astros
DJ LeMahieu, Yankees
Mike Trout, Angels
Carlos Santana, Cleveland
JD Martinez, Red Sox
Alex Bregman, Astros
Gary Sanchez, Yankees
Michael Brantley, Astros
Jorge Polanco, Twins
Throwing in some slightly inferior but still excellent performance for the bench, such a squad would hit at least 350 home runs and have an on base percentage of about .390, both unprecedented in modern MLB history. I ran likely combinations of hits and walks, plus the homers, and came up with a rough guesstimate that such a team would score about 1,350 runs, an astounding total. The record is 1,067.
How many runs would such a team allow? Very few indeed. This is much harder to estimate but let’s start with the fact that the stingiest team in the American League (I’ll stick with the AL to always keep the DH in play) was the Astros, who permitted just 640 runs to score. An All-Star team would be even better though, supporting Justin Verlander and Gerrit Cole with a better back end of starting pitchers, like Shane Bieber, Lance Lynn, and Mike Minor, and filling the bullpen with relief aces; my back of the envelope guess is this would subtract at least eighty runs. And of course our new squad will have slightly better defense and room for a defensive replacement for Gary Sanchez, who isn’t much of a glove. The Astros were a really good fielding team, to be honest, but we’d get a few more runs. Let’s carve off 100 altogether.
So our All Star team will score about 1350 runs and allow just 540. (That is roughly what I’d have guessed anyway.) Using the Pythagorean method, we know such a team would have an expected record of 136-26, blowing away the 2001 Seattle Mariners by 20 games.
Still, they don’t win every game, and 136-26 is against all other major league teams, who will collectively be .500 teams when not playing the superteam. Against a championship level team they’ll lose a bit more often, and could quite possibly lose a playoff series.