Happy to learn you finally have ordered a Serenity too ! Congrats, you will love it.
WRT your questions:
1) I've tried the Sportech version before ordering the wood version. On the weight difference (12.3 vs 14.1 kg), it's very difficult to notice these 1.8 kg weight gain while sailing it. However, I've noticed the lighter woody weight at the end of the day when I need to uphaul the board on top of the van's roofracks. While sailing, the wood version is a bit crisper and better absorb vibration when running across or pearling the nose through chop. The nose of the Sportech version showed more vibration amplitude (up and down "shaking") than that of the woody version.
2) no obvious speed difference between the 2 versions.
3) sportech version are long lasting : no damage when exposed outside for 10 months under sun, heat, cold, rain or snow. My woody is kept pampered in its dedicated boardbag and inside a garage during winter, but no damage so far except for the unavoidable few ding at the front and back of the hull while lugging her around on the beach between boats, piers, wall, rocks, tables, chairs or even peoples. Such a 15 footer is indeed long...
4) you should be fine with your current largest sail in 6-10 knots wind, no need for a jumbo sail unless you want a challenging ride in ultra light wind and want to smoke anything in sight on the water (a Serenity + 11 m2 sail beats another Serenity + 9.5 sail) !
5) for non-planing conditions (below 7 knots for me), the Serenity is the fastest hull so far that I have tested. Better than a Lechner or a Hobbie 18 footer. Above 7 knots, a dedicated planing hull is faster, no question. Selection of the right combo scenario in gusty 5-9 knots wind range can lead to interesting results as a planing hull is not always the fastest in a drag race (Serenity + 9.0 m2 sail against a large iSonic + 11 m2 sail).
6) I have zero experience with weed fins, so cannot be of any help or advise in this area, sorry.