My binder is pretty loose, I haven't worked out with it on, but I've been hiking with it and it was fine. I think it would be okay for me to do cardio in it, since I can easily take deep breaths while wearing it. Sport bras for me don't really work that well, I have to wear at least one sport bra with a tight workout shirt that has the elastic to keep your tits in place, but for really bouncy stuff I have to wear two sport bras and the tight shirt. I think my binder would be perfectly fine for me to do cardio in, and it'd def do a lot better job than my current set-up at holding everything in place, and I know I'd be able to breath well in it. I just don't do it now, because I only have one binder and I don't want to have to wash it even more often.
If I had a tighter binder, I wouldn't consider doing it.