Hi TFM,
Most applications of VSPs are on large vessels like ferries, or on tugs, where high speed is not as important as the maneuverability. Large tugs for tanker escort have maximum speeds similar to the speeds expected for tankers in those waters.
R/V Melville and R/V Knorr had top speeds of about 14 kt when they were fitted with VSPs.
However, smaller vessels can be driven at higher speeds. German minesweepers of WWII were able to reach 23 kt.
Regards,
Marty