Unless you have an actual weakness of the lung (have you done one of those tests where you blow into the pipe etc?) the chances are using one of those devices probably won't do much for you when compared to actually riding. If the asthma is an issue I suggest seeking a proper medical assessment.
But by and large the lung devices are a "gimmick" in the sense that they will give you stronger lungs - but usually it ain't the lungs which are an issue, oxygen intake per se is not usually a limiter.
Once again, check with the doc.
"Physiology is all just propaganda and lies... all waiting to be disproven by the next study."
"I'm not a real doctor; But I am a real worm; I am an actual worm." - TMBG