Hey mate i'm not sure you should go sanding your fins just yet. Yes the 2011 slotbox grub screws are too long and protrude beyond their recess when the fin is correctly fitted. This is an issue with the screws and not the fin so if this is indeed the problem you are experiencing then i'd be very reluctant to start sanding away at the fin just yet. My concern with sanding is that you might weaken the hold that the screws have on the fin, potentially making it easier to rip the fin from the box. You will also hit the same probs if you decide to experiment with other fins in the future.
It might be best to look at finding a solution with the screws. Some options:
1. Buy some 2012 screws. The 2012 Slotbox screws are shorter and have a thread that continues up to the head. see pic. top is 2011. bottom is 2012. Not sure what Starboard were thinking in 2011...?
2. Cut down your 2011 screws. The problem with this is you are removing a substantial part of the grub screw's thread, which could potentially create problems like stripping the thread from the box if you were to overtighten the grub or something like that. It might help is you reinforce the slotbox thread with a little epoxy. Cut the screw down to the correct length so that they dont stick out of the recess when tightened. With the fin in the box, put the screw in and turn it just enough so that it holds in the insert, blocks the screw hole and has a gap around the head of the screw for you to drop in a little epoxy into the recess under the head. Carefully drip in a few drops of epoxy into the recess and then tighten the screw to the normal amount required to hold the fin. Make sure that you do not have too much epoxy. You do not want it to squish out and cover the top of the screw or get into the hex. This would be bad. It would also be bad to allow the epoxy to pass the grub screw and glue your fin into the box. This wont happen if you are careful. Once the epoxy has gone off you can untighten screw as per normal. You should be left with an exact epoxy mould of your grub screw within the recess, hopefully giving your screws a bit more hold and support.
3. Leave the screws sticking out to drag in the water.