Question about writing for flute

How big of an interval leap is possible on flute? Specifically, is a sixteenth note leap of an octave + a perfect fourth (Bb5 to F4) at quarter note = 80 BPM possible?

Yes. Just don't do it too much. (Unless you're writing a study)

Almost any leap you can imagine is possible on flute.  The thing to be careful about is the relative sound quality between registers.  For instance, F4 is toward the bottom of the flute's range, which is naturally softer than the mid-range Bb5.  So leaping down from Bb5 to F4, the F4 may have a little trouble "speaking", at least in fast passages.  At a slower tempo (and certainly if they're quarter notes at quarter note = 80 BPM), the flutist will have enough time to adjust and make the F4 speak clearly.