A while ago I worked on feed forward from BSC St1 sensors to St2 in Z. I've installed this and it has been running on all BSCs for a while, but I have taken my time in logging it. Attached spectra show the improvement, where dashed is on (ground STS and St2 gs-13s) and solid is off. Almost an order of magnitude improvement over 10 to 25 hz. It does do slightly worse at 4-5 hz, because of gain peaking, but we are mostly into gs13 noise at that point anyway. I've worked some on other DOFs, but Z was the easiest to do with the worst performance. In X&Y the existing controls already mostly keep us in the GS13 noise, so the room for improvement is smaller.