In my opinion the answer is nuanced. "It's high time...to do something" in American English means that there is a need to direct attention toward a subject and that the speaker has been contemplating for a bit of time. Imagine if you will that you have been talking to a friend about a third person who is the apple of your eye. You've been going on for weeks and frankly your friend wants you do go ask him or her out. It's high time you asked him or her if she is busy on Friday. On the other hand come back has this sense of a future action, "let's come back to this later.." and in that sense there is a mismatch in time. But used in the sense of bringing up a topic of conversation I would say they are equal in meaning.