Exactly! And that's where I sense a nuance of difference between 'might' and 'could' (provided the context supports this reading):
"so that I might catch up" says that he gave me a channce to catch up - it doesn't imply that I did catch up.
"so that I could catch up" strongly suggests that I did catch up, ie. he stopped and waited until I had caught up.
Am I just imagining things or would you agree that this nuance is possible?