Can you not so fast Stormbringer?

No, you cannot use Not So Fast to counter Stormbringer.

Reasoning

According to the card text and game rules:

• Not So Fast can only counter a spell or ability that "chooses a friendly unit or gear."
• Not So Fast requires the spell or ability to choose a unit or gear that is friendly to you (the player casting Not So Fast).
• Because Stormbringer targets a unit friendly to its own controller, it does not meet the targeting requirement for Not So Fast.

Clarification on Targeting

• Stormbringer targets a friendly unit (which it moves) and a battlefield (where it deals damage).
• It does not target the enemy units at that battlefield. As clarified in RiftJudge FAQ #3073 and FAQ #656, Stormbringer targets the battlefield, not the units on it.
• Even if Stormbringer were considered to target the friendly unit, it is not a unit friendly to the player attempting to cast Not So Fast, which is the fundamental requirement for that reaction to function.

Note: I could not fully verify this ruling against the retrieved rules.