I'm not sure that's a valid question. The angular momentum simply won't go away. The mass is constant, just highly condensed. Neither is converted into 'spacetime', it's just that all we think we will observe is a tiny bit of rotating spacetime. (It might be a good idea at this point to define what you mean by spacetime.)

Again, remember, we don't have a laboratory-size black hole to study. It is difficult to think that the objects at the centers of galaxies, and the supernova remnants, and whatever it is that is generated by the super-energetic cosmic rays hitting our atmosphere, and maybe CERN particles, are not black holes; and if they are,the last two cases are going to generate a lot of useful information. Maybe we will even get a clue as to what the quantum properties of black holes are. It's a shame the name black holes stuck; gravitationally collapsed objects is a lot better description.