Mass-energy of black hole spins

This article describes a paper in which the spin of the supermassive black-hole in M87 was observed and its corresponding energy was calculated.

The figure given was 10^64 ergs of energy contained within the spin of the black-hole. By mass-energy equivalence, this corresponds to 1.1*10^40 kg. That would mean 5.55 billion suns’ worth of mass-energy are contained within the rotational energy of the black hole.

I find this extraordinary given the black hole weighs 6.5 billion suns, only slightly (proportionally) larger than the energy contained in its spin.

So, if an advanced civilisation were to extract all this rotational energy, would we be left with a black hole weighing only one billion suns, or are these entirely separate components? If the former, how does extracting a black-hole’s mass-energy through its spin comply with the conservation of information? Does the black-hole spin somehow encode a large amount of information about its components?

Finally, if large proportions of the mass-energy of black-holes can be stored in their spin, is there an upper limit? For example could you have a spin-black-hole where 100% of its mass-energy is stored in its spin?

Thanks