No, the scaling letters are a rough guide as to how effective the scaling is, with that much scaling the extra damage will likely be far higher than just 50.
However, the letters are only rough guidelines, and it is truly on a sliding scale, so just because something has an "A" scaling, does not mean that scaling is far better than a "B", the difference could be marginal.
However, I do not have a guide for exact numbers, but there is a weapons guide that tells you the exact scaling rating of a weapon at each upgrade path here:
Dedicated to digging into the game's lore. Bring your thinking caps.