I'm not sure how well that would work with the surfin boxes, so I'd be a bit hesitant to go far past 12cm, particularly if you at the same time put more load on the rear fins by making the front fins smaller. In any case I would try the 13 front fins (with 11 in the back) to see. That will already be quite a bit of difference relative the 16 front fins.
Smaller 2011: I'm yet to get on the 74, so I don't know so much. 57 cm sound A LOT, but remember the EVO 75 was already 56, as was the ET 74. Both those have a more parallel outline (which also goes for you q81). You can think of this as a wider board with the widest mid part shaved off. So on practice my GUESS is that the 2011 Q74 will not really feel bigger than E75, ET74 and previous generation boards in this size. But I can tell you more after sailing it. Just need wind...
The Q69 is 53.8, so quite a bit smaller and a more extreme board overall, I'd say. Ultra quick and epic for wave riding for lighter sailors, but I'm not sure how it will handle a bigger guy. Need to get some heavyweight on mine. Again though... wind. Might have to go to Demmark to find some.