First, you are right that rotation on the 17 Sport is only controled using sheet tension. The boomlet simply allows you to adjust the sheeting point on the foot of the sail and thereby gives you some control over twist in the sail. In my opinion the sport setup compromises control of the sail in favor of making it easier for the crew to get from side to side. If you are going to invest in the square top I suspect you want to be able to adjust it accurately and have control over rotation. The guys at Calvert will be able to guide you here.