I have never made a reflex before. It's something I am working towards, but saving for a shorter stave of around 64". I am reluctant to sacrifice any of my 66-72" staves for what, for all intents and purposes, would be an experiment on my behalf.
Personally speaking, I would aim towards a 66" (not so long so as to reduce efficiency, not so short as to reduce reliability) slightly reflexed (1 1/2" only) flatbow with real skinny tips.
I am not a real big fan of the the look of Andaman/Mollegabet style of lever bows, though they reportedly shoot real fast, and is probably more suited to lesser woods where width is a virtue. (Lesser then Osage, is like all other woods, barring except maybe Yew)
Maybe more of a Holmegaard style with a gentle limb taper for like 2/3 limb length, then an increased taper to pencil thin tips for the last third. The inner 2/3 would do most of the work of course, so the outers would be mostly stiff. Same principle as a lever bow, but to my eye, much more pleasing. If you limit your handle/fades to 6" it gives you more working limb.
The Mass Principle described in TBB4 would contribute to the bows speed also. Osage does not need to be real wide, a little side tillering can remove mass with minimal poundage......