No idea if it is classified or not, and tbh 50 years after its retirement I would suggest its limitations cannot be regarded as a secret.
According to
this paper from 1953 the X-5 was designed with a load limit of 7.33g, which was I think, more or less industry standard at the time. Remember this aircraft was built to investigate the effects of variable sweep and as technology demonstrator to prove the concept of Bells sweep system, not as a prototype combat aircraft.