They are not quite the same meaning. So to speak means 'as the saying goes' or 'in a manner of speaking', most useful when it indicates that an expression is not to be taken literally. We still don't have our heads above water, so to speak. That is to say means 'in other words', 'or more exactly', 'or to put it another way' or 'I mean'. We're only human -- that is to say, we don't always think with our head.