How efficient is stock for protein extraction?
From Harlod McGee's "On Food and Cooking: The Science and Lore of the Kitchen":


  The muscles that make up meat are mainly water and the protein fibers
  that do the work of contraction, which are not dispersable in water.
  The soluble and dispersable materials in muscle include about 1% by
  weight of collagen, 5% other cell proteins, 2% amino acids and other
  savory molecules, 1% sugars and other carbohydates, and 1% minerals,
  mainly phosphorus and potassium. Bones are around 20% collagen, pig
  skin around 30%, and cartilaginous veal knuckles up to 40%. Bones and
  skin are thus much better sources of gelatin and thickening power than
  meat. However, they carry only a small fraction of the other soluble
  molecules that provide flavor. (pg 598)


Accordingly, that 7% (5% + 2%) may be considered the upper bound for protein extraction, which will obviously depend on the specifics of the meat and cooking process. McGee then goes on to discuss the proper process for extraction, starting with cold water gradually heated:


  The cold start and slow heating allow the soluble proteins to escape
  the solids and coagulate slowly, forming large aggregates that either
  rise to the surface and are easily skimmed off, or settle onto the
  sides and bottom. A hot start produces many separate and tiny protein
  particles that reamin suspended and cloud the stock; and a boil churns
  particles and fat droplets into a cloudy suspension and emulsion. (pg 599)


This also indicates that the the amount of emulsified fat will depend greatly on on the cooking process in addition to the fat content of the original ingredients. (I haven't been able to find much on differences in the extraction process for different animal fats.)