Hi, Yes it would be the NBI Clearance. And yes I would recommend to wait till your exam/interview, if they would ask for it or no, as I felt it was more subjective depending on the immigration officer handling the review of your documents.
Should they ask you for it, the officer would hand you over a letter where you have 30 days to comply with the required document. You may request for it to be extended. for NBI clearance application, my parents applied for theirs in the same building of our consulate office in Yonge and Eglinton.
Theres an office in the 4th floor where they take care of your NBI application that has a rush option for 2 weeks. the NBI clearance rush cost $100, plus finger printing (I believe $35/ea not so sure).
** There may be other agencies who can do the application for NBI. I'll update you should they receive their clearance in the next week or so.