RE: Gaastra 8.3 Total Flow F1-11
An 8.3 rig is for sure going to give you some earlier planing performance over your 6.5 m2 Pilot, but I'm not sure the Total Flow
F1 Race sail is going to be just what you need here.
I've sailed one a couple of times, and it was a pretty nice sail, but as I recall, the people who loved the Total flow were guys that took the time to get the right mast and fitting under the head spar. Not just any mast, and you have to have a fitting that fits the head spar tightly.
It's also not the easiest sail in the world to rig and tune.
If you got it all figured out, then the TF F-1 was quite good.
If you didn't get it figured out and have the right mast and fittings, then it was somewhat disappointing.
If the price for the sail is really good, and you can find a spare head spar and the right mast, go for it!
If not, perhaps a good 8.5 m2 free race sail would be alot easier on you.
As far as extending the boom, yes, when you get out to max. extension, the loading on all the other parts of your boom goes up, so the weakest link in the chain will be more prone to failure.
I like to see at least 10 cm extra in a boom because if you add more than the recommended downhaul (to flatten the sail or make up for mast/sail incompatibility) you could extend the max. clew length by as much as 4-5 cm.
Not much in windsurfing is worse than a rig that you can't outhaul fully as the sail stays drfity, baggy, and will load up too much.
So, a longer boom for the Total flow would be almost a "must".
Hope this helps,