This is pretty cool, but from a hard SF perspective, evaporating a non-trivial region of existence is going to release immensely more energy then a little bit of hawking radiation.
Maybe if the collapsed matter/energy is shunted somewhere else it would help in the whole conservation of energy thing. Maybe these weapons create nasty explosions in another dimension. Might attract the wrong kind of attention...
The second effect of obliterating a fixed radius of reality if in an atmosphere would be a massive shock wave as the vacuum collapses. This would resemble an explosion whose energy would depend on the volume of the destruction.
For fun I threw some numbers at it. A 1 km radius sphere has a volume of 5.24×10^8 m3. A vacuum has VERY roughly 2kJ of energy per m3. So, ~ 1x10^9 kj - which is roughly .2kilotons. A good boom. Go to Comment
Great - but I think it could go a little further to work properly.
In addition to the musculature of the face, changes would need to be made to the underlying bones. Bone would need to be removed in multiple places to allow for ratio changes between the various facial feature to allow for expansion or compression. In these joints would be placed either implanted servos (bad for lots of reasons) or tissue that could be caused to swell or contract as needed. This would allow for dramatic changes in appearance but would have the side effect of reducing the structural integrity of the face.
Another side effect of this is the facedancer loses their original face. Their resting face would be an odd one - contracted with with features a bit too close to be natural. So, they would always normally be acting.