Hmm...it's really subjective, isn't it? If they seemed very welcoming and understanding, I'm sure texting them every so often wouldn't be a problem. Maybe once a week (ish) until you feel certain that Jasper is really doing well?
I'm sure they can understand your love for Jasper and that you need that reassurance that he will be okay. At the same time, you don't want to overwhelm the new owners with too many calls or texts--they are probably ecstatic with their new addition, and they want to focus on him :) I can imagine how tough it is, and I think it would be good for you (and them) to check in regularly until you feel confident that Jasper is happy and healthy in his new home. I could also see them understanding that you are checking in to make sure that this decision is for the better health of Jasper--if you don't check in, you won't know if this change has been positive for him. And the whole point of this change was to make sure that Jasper is a happy and healthy cat...everything he deserves!