21DS115 is awesome driver, but I see it as such because NSW6021 is more than twice expensive than B&C one. You have it much closer, albeit Eminence not being on hand. 21DS115 definitely cannot do 20mm without ton of distortion and mainly power compression. You do not want to go above 17mm with this driver. I suggest that with up to 1000-1500W it is cheaper adequate choice, and at or past 1500W, it will start to lag most likeky.