There have been quite a few reports on the forums of reports being sent back for clarification. It says nothing about the probability of getting an award - it just means there's something in the report that DWP do not find clear. I'd give it at least a fortnight to see what turns up in the post.
At this stage, I'd leave well alone and let the decision be made. PIP is about the functional effects of the claimant's health conditions and disabilities on the various activities, so an additional diagnosis is fairly unimportant unless it changes these functional effects.
You can always report the new diagnosis as a change of circumstances if you think it will lead to an increase in the PIP award. Once you have the decision, you will have a much better idea what to do.