Actually yes the upgrade is a cheat, because it didn't exist. Type II through VIIA did not have superchargers. Type VIIB on came with them from the shipyard. The standard speed in the game is the actual speed the subs made, WITH the superchargers. Anything faster is unrealistic.
VIIA: 17 knots
http://www.uboat.net/types/viib.htm
VIIB: 17.9 knots
http://www.uboat.net/types/viib.htm
VIIC: 17.7 knots
http://www.uboat.net/types/viic.htm
IX: 18.2 knots
http://www.uboat.net/types/viic.htm
IXB: 18.2 knots
http://www.uboat.net/types/ixb.htm
IXC: 18.3 knots
http://www.uboat.net/types/ixc.htm
IXC-40: Listed as 19 knots, but with the same 4400 horsepower and the same displacement, one of the speeds listed has to be wrong.
http://www.uboat.net/types/ixc40.htm
Still, no official source lists ANY u-boat as making 20 knots, even downhill.