Hi sv607. I actually like the standard rudder on the bluefin for paddling in calm conditions. I find it responsive enough, it creates little drag and sheds weeds well. However, like you I didn't buy the bluefin for calm conditions so that rudder had to go.
I can't comment on the performance of the Orca rudder. It looks very similar to the Fenn unit. However I can say that the Fenn downwind rudder transformed my bluefin. It is noticeably more resposive and has far more bite for surfing. I am much more able to surf diagonally on a wave now.
Current boats
Epic V10L Ultra, Epic V9 Ultra, Fenn Bluefin, Nelo 600, 550, 520 & 510, Expedition Kayaks Azure, Mirage 732.
Previous boats
Spirit PRS, Fenn Swordfish, Fenn XT, Fenn Swordfish S, Think Zen, Epic V10L Club, Carbonology Sport Boost LV, Carbonology Sport Boost X LV, Fenn XT double.