Brady's record without Belichick, 37-20, 5 playoff wins, 1 Super Bowl.
Belichick without Brady, 84-103, 1 playoff win. Belichick with Brady, 249-75. 6 Super Bowls.
Obviously a lot goes into why, Cleveland years are tough to judge, rebuild post Brady was tough, Brady did go to a solid Tampa Bay team, etc. But still, those records aren't rounding errors, it's a massive difference. Standard deviations of difference.
Bill is the best defensive mind in the history of the NFL. And he's the best coach in the history of the NFL. But Brady undoubtedly elevated his record, because it was a perfect match. (Bill ran the team and the D, Brady the offense)
Without Brady, Bill didn't come close to winning a playoff game with NE. Without Bill, Brady won the Super Bowl less than 11 months after he left the Pats.