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.

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.

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.