Stafford is the best QB the Rams have had since Warner.
But, whereas Kurt was on the cautious side, Stafford is on the more reckless side. Both are simply wired that way and you don't ever want to try to re-wire those guys.
The answer is in play calling. Don't call plays for Stafford where a conservative approach is needed, but the play itself is designed that has some risk involved because Stafford will always lean towards the risk.
Starting after the 3 game skid, there was an observable difference in play calling. No more 5 yard out/choice routes on 3rd down from your own 20. Unless the Rams are in a desperate situation on the scoreboard, you give Stafford conservative calls in risky territory, and things will function much better. In the middle of the field? No holds barred. Turn him loose. He'll still take a riskier way than many would like, but he'll win well more often than he loses on those plays.
We don't win the Superbowl without the Stafford that is, rather than the one many would like him to be. But McVay can have a mitigating effect to maximize Stafford's best attributes simply through play calling.