Additional processing times:

This is a very frustrating part for lots of doctors, especially those coming from Muslim countries. This can potentially harm your career prospects and taxing financially.
The Additional processing time is variable and different for different people. It can take few weeks to months. I suggest not to hide anything from the visa officer, even though if you think it is trivial and does not matter. US embassy can track the records well beyond 12 years (my personal experience).
There is no guarantee if you had additional processing once that you will not have it done again. I applied my visa from London embassy, UK and waited 7 months for the clearance. During this period I could not go for CS exam and hence lost my Match. I applied from my country of residency (Canada) once I thought I had clearance after 4 months of clearance confirmation from State department. I was surprised that I required the processing again. I hope this time it will be only few weeks. I will keep updated.
The key here is that you have to plan really early and anticipate that you will require these additional processing time. You have to give your CS early, may be when you are going for your step 1 or even before that.
NRMP will remove all the FMG's who do not have passed the step 2 CS before certain deadline, usually the last day of December for next years match.