I think one of the reasons a BF baby *may* sleep better (or appear to sleep better) if a bottle of artifical infant milk is given is because the baby has to work so hard to digest it. BM is easily digested & is so with in 2 hours. Where AIM takes much longer and is harder work on the digestive system of a BF baby.
So while they may appear to be having a better nights sleep it may actually be because their little body is working so hard to digest the concret in their gut that it shuts down so it can work on that & that alone. But you may also end up with a very unsettled baby that just doesn't know how to deal with sucha heavy tummy.