There's approximately 15% chance that speed increases by 6 giving you you maximum of 11. Since it's possible to have a grey mod with a 6 speed secondary, the maximum secondary speed a green mod could possibly have is 12.
The other possibilities are approximately 30% chance of +5, approximately 30% chance of +4 and approximately 25% chance of +3.
errors on the probabilities are difficult to state accurately but are in the region of +/- 3% since they come from peoples limited tracked results and not from any official sources.