The "kit" they sell separately is a set of plates that bolt onto the rear arm where the endlink attaches. It turned out in a few cases that the weak link was not the endlink but the frame attachment itself. The kit provided a plate and bolts that would line up with the 8th gen frame mounting point to add strength.
The kit came with the progress bar for free and I don't know if they would line up on your 9th gen. The eibach bar looks like it fits very well and it is not that much of a size difference. You also needed new frame brackets for the 8th r18 because of how the bar fit onto the "body" of the car underneath. The Si had longer brackets to start and most parts were made for the si and the r18 crowd ran adapters. Some sway bars came with extension hardware.
Get the eibach, it looks like less of a hassle and you know it will fit nice!
The kit came with the progress bar for free and I don't know if they would line up on your 9th gen. The eibach bar looks like it fits very well and it is not that much of a size difference. You also needed new frame brackets for the 8th r18 because of how the bar fit onto the "body" of the car underneath. The Si had longer brackets to start and most parts were made for the si and the r18 crowd ran adapters. Some sway bars came with extension hardware.
Get the eibach, it looks like less of a hassle and you know it will fit nice!