Go the dark side - get a foil
Tried one a few times and in light winds you can feel the difference.
If that can help the initial question: at 85-90kg, a 7.8 isn't going to be enough on a light wind board to plane in less than 14-15kn I'd say.
Amongst a few other things, I sail a Fanatic Falcon Lightwind (equivalent to the JP) for the past 2 years now and weighing the same as you, I need a 9.4 with cambers to get this beast going in 11kn. It needs a great deal of pumping too but once it goes, it doesn't stop! In 13kn steady on a 8.5 Lion for example, it becomes more manageable and friendly.
Anyhow, after 2 years of "fun" with the 9.4, I offered my right knee and a shoulder a nice case of tendinitis due to excessive pumping (I'm 54, not in my 20's anymore 
). Conclusion, I'm ready to put a foil under the Falcon so I can happily sail in 11kn on a friendly 7.0 or smaller. Christmas is approaching and I feel Santa is going to be kind to me. 
So, I'd agree with Photosbykarlo and consider foiling! Heard it's fun.
One sailor here about 65kg & 7.0 & 8.0 NCX main still used on SBUS147.
So on a JP SLW anything from 6.7 up and choose a sail size that suits the conditions.
If price is no objection, would you buy a 2018 SLW in FWS or Pro. There is 1kg difference - which is more durable?
These are all now foil ready.
Select to expand quotedhat said..
If price is no objection, would you buy a 2018 SLW in FWS or Pro. There is 1kg difference - which is more durable?
If price is not a factor, i would get the lighter. I remember reading that 1kg or 2kg difference is not important once planing. My experience is that it is making a difference:
If the water is 150 yards away from your car, lighter means less energy spent carrying the stuff in and out.
Lighter boards don't stick to the water as much and tend to release faster, which means less pull on the arms and less pumping which also saves energy.
And I think the Gold would be noticeably better than the FWS for that light feel under the feet = more lively and fun to ride.
Finaly, I don't think one is more durable than the other. All modern boards dent easy and crack on impact. Although hard to prove, my experience told me that Starboard's in Carbon tend to be slightly more fragile than the equivalent with Goya's. Don't know about JP's.
Thanks. I'll have to weigh up saving EUR250 on the board and putting that for instance into a better mast - maybe I'll get the FWS after all. In your experience is weight saved on a board the same as weight saved on a mast, sail or even bodyweight? The Gold is outside my price range - the Pro is only 1kg lighter.
Slightly different topic but somehow related to weight, would I be better off with a Cheetah 9.5 (no cam) or a NS SL 9.3 (2/3 cam)? I've never had any cammed sails.
Imo once you go over 7.5 you need some cams to hold some depth in the sail, without them the wind has to fill all that heavy sail cloth before you start getting drive & then there the stability increases so you might get another few knots range (depending how the reat of sail is designed).
As long as you get narrow sleeved 2/3 cam you will be fine, you arent falling off as much & throwing the sail around like in stronger winds, plus you have plenty of volume to uphaul. Boom length & luff width affects waterstarting more than just having cams.
