I think the extra width of the iS 150 is more critical to early planing than the weight difference, plus you can use a longer fin which will also help. The extra volume of the 150 may help a little, but it is somewhat insignificant.
At your weight, to plane in 7 knots, it will take a 12.0 sail and a formula board, so don't get your hopes up. I have an 11.0 and a formula board (last 8 years) and weigh 78kg, so I have a pretty good idea of what it takes to plane. I sail were the winds are variable (up and down), but with pumping, I can get on plane in about 8 knots. At 10 knots, planing and speed increase significantly, at 6-7 knots, I fall off plane.
With the 150 and an 11.0, you will not have much fun unless the wind is at 10 knots or more. You may be able to plane in 8-9 knots, but you won't be able to point or bear off much, so you will be stuck with just beam reaching. With the wind over 10 knots, you will be able to point moderately high and run off the wind about 110+ degrees with good speed & much deeper with more wind. It's a lot more fun if you can take advantage of the large board and sail all over the lake/ocean rather than staying in one place.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.