Nel has boxed Roux into a corner - hence the emotional aspect to his objection. I think he was further irritated when Nel responded by saying the reason why he knew so much about the case he was referring to, is that because he was the Counsel involved and was arguing against the motion.
I believe that it matters not whether Judge Masipa grants the referral. Either way, the Defence is severely compromised as by stating that they would be bringing yet another Witness, they are saying that due process should not be followed.
Essentially, Vorster's testimony has created the requirement for Nel to ask for a referral. Arguing against the referral by saying 'Oh, well we are bringing someone else to court and we might have to have another referral if different aspects are highlighted,' shows a lack of preparation for this situation, which I find staggering.
In my personal opinion, Masipa will say no, but will declare that the defence cannot rely on Vorsters testimony.