It's been already implemented in GWX since v.101, you need to change the values for AheadSlow in the subs cfg files, when you order silent running the sub speed is set to that.
This are the values from GWX
Uboat 21 :
AheadSlow=0.24
Uboat 2A:
AheadSlow=0.30
Uboat 2D :
AheadSlow=0.26
Uboat 7B:
AheadSlow=0.25
Uboat 7C:
AheadSlow=0.26
Uboat 9B:
AheadSlow=0.28
Uboat 9C:
AheadSlow=0.28
Uboat 9D2:
AheadSlow=0.30
Ref
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