I have the older Mibox which doesn't have 4k Chromecast. I can tell you that it is not laggy at all. It runs absolutely smooth. I watch Netflix on it, no problem at all. Also, why do you need the storage? If you want to download a video you'll have to do on your mobile/pc - Android TV is not well suited for such stuff, no matter which Android box you have. It is suited for Youtube, Netflix, etc., not downloading.
There is one issue that I have faced with Mibox: The bluetooth is problematic. For no reason, my bluetooth speaker would start stuttering, or when I connect the bluetooth, the video quality will drop.