Glad to hear Lickorish is feeling better. My kitten coughs like this every now and then as well. The last time I called my vet, they told me that if it stops within a few minutes not to worry about it. I'm second guessing her now though...
If I bring him in, even if he's not coughing, were the X-rays able to support the diagnosis? If he's not doing it right then and there, should it still show up?
Also, was Lickorish's cough something that was "cured" or do you have to continually manage the asthma?
Bottom line is that I need a new vet. I love her to death, and she's amazing, but her staff sucks, and they're always booked weeks in advance and seem to try to discourage you from bringing your kitty in when they're sick so as not to screw up their appointment schedule for the day. I always get the "just keep an eye on him" line. Maybe I overreact. I probably do, but I still think I need to find a vet that makes themselves reasonably available for things like this and staff that doesn't make me feel like a burden for calling in.