As for jibing the Serenity, I don't bank the top of the rail all the way down to the water (slam jibe) because it does make the board very slow and very tippy. I prefer to bank the board slightly on her shoulder and make a nicely flowing large turn : speed is kept all the way to the new direction and she is not too tippy.
As for sail and fin size, I'm only using the stock 70 cm XXL Drake fin with a Reflex II 11.0 m2 sail. This is the combo which is getting the most fun and racing challenge for me. Yesterday, we had a very gusty and shifty wind on our lake : 1-10 knots offshore. I could have been with a Formula + 11.0 m2 sail (25% planing vs 75% schlogging time), Phantom 295 + 8.6 m2 or Serenity + 11 m2. Guess what ? The Serenity + 11 m2 combo was a killer to smoke everybody else out on the water, except a couple of class C cats and flying Moths. Huge fun for more than 4 hrs. Not the quickest in the strongest 10 knots gusts for sure while foiling da big Bertha, but still the fastest around on the average 5 knots of wind. I'm still in awe with her after all these years...! Best ultra-light wind board ever made, period.
Did I tried my Serenity with a Reflex III 8.6 m2 sail and a Deboichet R16SL 54 cm fin ? Yes.
Did I get fun ? Yes.
Did I get the most fun out of that combo ? No.
Do I want to repeat that ? No. Main reason because I did not get the high thrill and racing challenge as with the larger fin and sail combo.
However, this is not to say that you must have to use your 10 m2 sail to get the most fun with your Serenity. Bigger sails are heavy and a pain to uphaul, no question. I'm sure you do have fun with your 7.5 m2 sail and the shallow Drake 40 cm fin. I also have used this shallow fin a couple of time in the past but I much prefer the big hard 70 cm fin which is more direct under foot, more racing oriented, more biting upwind, more exciting to ride and to foil. I guess it all depends on which features and results you are ready to make a compromise and on which one you are not. This is why this board is still unique to fine tune, from a easy going plug & play serene glider up to a challenging racing beast.