We were in a similar spot one week ago!
At first our son (born 3 pounds 13 ounces at 31 weeks) was not tolerating the HMF well, but after a few days he seemed to get used to it. As we approached 33 weeks, we agreed to continue using the HMF when he was tube fed but to also continue working on breastfeeding. Our plan is more or less to stop the HMF when he's fully breastfeeding.
Over the last week - he's 34 weeks today - he's really figured out breastfeeding and he's now getting about two thirds of his milk by breast instead of gavage. When we go home in about a week, we'll probably be exclusively breastfeeding. If he doens't gain enough weight like that, we'll consider fortifying.
We talked a lot to our lactation consultants and neonatologists about it. They had several thoughts. First, the HMF has not only extra calories but also extra minerals that the baby misses out on. The baby can get extra calories if you give him mostly hindmilk, but maybe not enough calcium.
The lactation consultants advice was to use the HMF, but to also use hindmilk. The hindmilk as more digestive enzymes that help the baby digest the proteins in the HMF.
We did some research online and it looks like many nicu's stop the HMF around 1800-2000 grams. Zephan's just over 2000 today, and as I said, we're stopping the HMF little by little as he takes less by gavage and more by breast.
We had concerns, but in the end I guess we decided that as long as he was tolerating it, we would use the HMF until he was breastfeeding but that we would not suppliment after that point unless there was a clear reason that he needed it.