Birch bark won't get you any draw weight gain Chuck, at least not the thin stuff I've used. I think Dakota Kid's suggestions would be one way to bump it up. Also a combo of those options and a good heat treating of the belly while adding some reflex could gain you
5-8#. When I do heat treat osage I find it responds well usually in the draw weight gain dept. Mostly though I will do it when I have the limbs bending how I want but having only been pulled to about 6" short of the desired full draw. For me that means heat-treating
when I have it pulling 18"-19" for my 24"-25" draw length. If it has been well exercised and worked often at or beyond the full draw
length the heat treat/added reflex gains are less. IMHO, milage may vary as they say.