Well I remember more than one guy on my Swinderby course who had to fly in the canvas 'G' Helmet as they didn't fit with the helmet as well so nothing is new!
My point re Prefect is simple. It is up to the contractor to provide suitable equipment for MFTS. If there is a problem with the Prefect and pilots unable to fit in then is is an Ascent issue, and then a contractural issue between the RAF and Ascent.
In the event of a forced landing, a bone dome will provide protection to pilots from injury. In the event of an abandonment it will protect the pilot is he or she is unfortunate enough to impact the airframe on the way out. These are indisputable facts and I wouldn't want to be at a Safety Action Group where I was asked to put my name on record as commending the removal of either item. Picture the scenario if there was a further incident whereby it was alleged that either of these would have saved a life/prevented serious injury following their withdrawal, and the subsequent hunt for those who made the decision.