Cambridge English Scale asserts that, although the exam is targeted at Level C1 of the CEFR, it also provides reliable assessment at the level above C1 (Level C2). See here.
The Statement of Results also specifies it:
"Candidates achieving Grade A (between 200 and 210 on the Cambridge English Scale) receive the Certificate in Advanced English stating that they have demonstrated ability at Level C2"
I usually state that I am C2 in my CV, but I want to know to what extent is this a valid official certificate or if I should take an exam targeted at C2.