I'd say take it. The 821 is only about 1-2 THs slower than the 841 though again, remember you cannot mix different models on 1 controller.

Regarding the mention of 3rd party firmwares for s9's - keep in mind that most charge you a couple % usage fee that is taken by the miner periodically changing to what is called DevPool. The Avalon's have no need for those 3rd party firmwares because they already support an extensive list of built-in performance tweaks to change clock rates, chip voltage, fan speed, operating temp, etc.