From the little information there is out there on the S-class, they all seem to mention a figure of 1 to 2 Knots faster both surfaced and submerged for the S-Class.
Take this as one example:
http://en.wikipedia.org/wiki/United_...lass_submarine
"Speed: 14.5 knots (27 km/h) surfaced; 11 knots (20 km/h) submerged"
EDIT: Or this
http://www.hazegray.org/danfs/submar/ss123.txt
Speed:
Surfaced: 14.5 k.
Submerged: 11 k.