Mick,
Further to your fin problem.
The SS 118 comes standard with the 42 cm fin, probably aimed to work with 7.5m + sails. For a 6.2m sail, Select fins suggest a 33ccm fin for this 65 cm wide board when used with a 6-ish sail:
http://www.select-hydrofoils.com/en/selector/speed-slalom-race/.My 36 cm fin works well with my 7.5m sail, but occasionally, I might use a 42 cm fin if I'm having trouble planing. As stated in my previous post, I would also use the 36 cm fin with my 6-ish sail but prefer a fin of about 32 cm if I'm fully powered.
Also, the straight-up-and down standard fin is more difficult to gybe than a fin that is slightly curved back. Using Select again:
www.sunshinecoastsailboards.com.au/Components/Board-Components/Fins/Select/Select-SRide-Pro-Series.html I'd expect that you would feel the difference and other fin brands may offer a fin even more swept back (in passing, have you ever used a 42 cm fin with the Carve?).
Compared to your Carve, the SS should plane earlier (given the same sail and fin size) and glide better during the wind lulls. This comes about because of the width in the centre of the board and the tail width, too. Depending on your weight, both boards should start to become a handful as the wind whips up the waves and the water becomes very choppy. I think that you should be able to hold onto the Carve for a bit longer than the SS because it is narrower all through.
The SS will definitely be harder to gybe. You have to go into a gybe close to full speed, use a wide radius and really bank the board over to keep it going so as to come out on a plane. There's nothing wrong with stalling through a gybe as long as you don't fall in (in this, I take my own advice). It takes maybe 2 good sailing days of good wind to master the SS in gybes. Also, you have to remember that JP aim the SS at upper-level intermediates and higher, because the SS is close to a race board.
I would HOPE that the SS is faster than the Carve, but you need to race against the JMan to gauge the speed of the Carve vs his SS.
Hope this helps.