Nice discussion going on here. This is a very complex issue since there are a lot of variables at play. Two examples below to illustrate that complexity :
1) 105 kg mate on Carve 161 + freerace 9.5 sail and myself 65 kg on Kombat 86 + freemove 6.9 sail. Drag racing side by side in steady onshore 12-18 knots of wind. Below 12 knots, none of us can start planing. We can both start planing at the same time with 12 knots winds and well powered-up, we were equally fast.
2) 85 kg mate on iSonic 122 (Drake Slalom Pro 42/44/46 cm fin) + Racing 7.8 sail and myself on iSonic 76 (Drake Slalom Pro 32 cm fin) + racing 6.6 sail. Drag racing side by side in gusting offshore 15-20 knots of wind. Well powered-up, he was a tad faster than me but way faster once we hit a ± 15 knots lull. Enter a 67 kg racer on Falcon 72 (Tectonics GW 36 cm fin) + 6.7 racing sail and this guy beat us on all point of sail with stuning planing ability in lower wind range.
1) match board size with sail size according to your weight for the conditions.
2) a wider board size with a fairly large fin is faster in gusting offshore winds.