I agree with master Tip

To answer your questions, no you would not be disappointed with an O2 sail and yes I believe an 02 can be as fast as a Switchblade.
I've been using only cammed sails for about 15 years till I got my 5.9 O2, I was really surprised how good it is. I have done over 35 knots with it on a freeride board (100L Kinetic Gladiator). I now have a 7.8 Switchblade which is also great for all the reasons Tip said, I use this sail for crappy days on a 111L Futura and the 5.9 O2 for very windy (rare) rough days on my slalom board at Yorkies Beach.
Is your 6.6 a Koncept, to give you an idea I spent an arvo chasing a mate (far better sailor) all over the Swan, he had the same but one size smaller board to me, I had the 5.9 O2 and he had the 5.8 Koncept, really there was bugger all difference speed wise. Before I get shot down, of course the Koncept is a faster sail, but I have found on less than ideal conditions the O2 will hold it's own.
I use a 6.8 Loft Blade on my slalom board 90% of the time, great sail but it will be a toss up when I replace it between another Blade or the Switchblade as I believe most of us in the real world don't need race sails.
Not knowing you, how you sail or your spot, I'd lean towards the Switchblade although you would be very happy with an O2 also.