I can't speak to the 161, but with your weight, your range should be much greater than 15 knots, even with a 10.6. However, if you have the Drake fin that came with the board (I think it did), it is best for lighter winds. I have a 160 and immediately went to a Debouchet R13 70 and at 167 lbs, I can manage the board in up to 18 knots with a 9.2 Maui Sails TR race sail. However I am much more comfortable with an 8.4 or 7.6 if it's around 20 knots. I have even gone down to a 6.5 in 25-30 knots for a couple of races, but that was survival mode for me. These days, if free sailing, a never go below my 9.2 on the formula board since I change over to an iS 111 with an 8.4 or 7.6 when it hits 13+ knots.
Regardless, nothing smaller than a 65 cm fin and a 9.2 should give you control in over 20 knots. It DOES take practice. Also, keep in mind that most formula sailors with big sails are sailing upwind and downwind with little reaching.
I have a friend that is on a HWR, weighs about 190 lbs and sails his formula board in 8-25 knots, using sails between 11.0 and 7.6. He only uses a 70cm fin. He only sails his formula board and never goes down to anything smaller.
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.