I have only tested my speed once, going 29 knots on the following setup:
Isonic 127 Carbon, 2011
Tectonic Talon 50 fin
North Warp F2011 9,3
Open water but surroundet by islands so only small chop.
Speed was recordet on a Garmin Forerunner 910XT
My guess is that your fins are a bit too big for a 7,6.