I lost 57 lbs before surgery (I think about 15 of that was on the two-week pre-op diet, though - so 42 lbs before the pre-op diet).
you'll likely lose on the low side the first month after surgery, since for most people, a lot of that huge drop the first month is water weight, and your water weight is long gone. Mine was, too. I lost 16 lbs the first month.
after the first month, I don't think it makes a difference if you lost weight pre-surgery or not.
surgeons usually quote you the average weight loss (60-70% of your excess weight), because statistically, that's where most people end up. But a statistic is a statistic - there are always people who are going to lose more or less than that. If you really work at it, you can do better than that. I almost never went off my plan and lost 100% of my excess weight.