This is another "recommend me a board post"
I'm returning to the sport after a long layoff and have just bought myself a JP supersport 100 (with 4.5m3 and 5.8m3 sail) but on days like today 8-12 knots I keep watching and thinking "if only there was another 5 knots.
I'm 78kg and intermedite skills and will sail almost exclusively on Lake Macquarie (Coal Pt).
Am I best to go up a sail size - I've promised myself a 7m3 sail for Christmas - and leave it at that or do I need a larger board as well? If so, which one/s?
Big boards are designed for 7 plus sails l dont think anything smaller will perform that good no matter the wind strength! 
I had the same dilemma and invested in a 121ltr Starboard Futura and have a 7.7 V8 for the light days. I can get planing in about 10 - 12 knots. I weight 75kgs. Its given me at least 6 extra days of sailing this summer than I would have had otherwise. If I could afford it, I'd pop for a Tabou Speedster 125ltr, but am pretty happy with the Futura. I sailed almost exclusively on a 5.5 / 106ltr set up for the last few years, but decided there were too many days where it was marginal and some bigger gear would be ideal. So built up with 2nd hand stuff over winter.
100 l board is under-utilized with a 5.8, you could go up to a 7.0 and still be comfortable. That would be cheaper than buying a new board. In the future you could get a bigger board, use the 7.0 on both and get an 8.0 for lighter days.
Hey Brewman, my weight and skill level is near identical to you, and I too was hunting either some huge sails or a light wind board for those 8-12 knot days that seem to frequent. Anyway, I already had a 111 litre Futura, which I can get up and running nicely in 12-14 knots with an 8 meter sail. But for my skill level that's a big sail on a smallish board. So I hunted around and got a used 136 lt Super Sport. I figured it would be a big old slugger and just cruise around and be good for the kids and sight seeing. But mate, it hammers along and gybes beautifully. It doesn't feel big at all, yet gets up and goes much sooner than the Futura. The biggest sail I've used on it is a 7.4 Loft freeride sail (the one in the clip) and can get her up and running around 25 knots pretty easily in 12 knots of breeze.
This clip is about as dismal a breeze as I'd hope to go out in, yet the JP SS / 7.4 combo had her going nicely- albeit with a few pumps and a little boucing...
