Hi there OS.
Boeing used to recommend F20 for takeoffs when the possibility of windshear existed. We questioned them and they offered a NTO for the use of F15, but not F5. I believe they changed the manual after that.
We keep F20 available for some VIP airports with short runways or close obstacles.
Interesting that you use F15 as standard, why? We have 2 schools of thought here, some prefer F15, some F5.
Mutt.