I submitted my application with LoE - mentioning that I have not received the FBI PCC. Later when CIC sent a Additional Document Request (ADR) for the same, I raised a Case Specific Enquiry (CSE). By that time, I had already re-applied for my wife's PCC (3-4 weeks back) and had couple of correspondences (emails) with FBI. So, in the CSE, I mentioned my situation and requested for time till end of Nov, to which they agreed. They gave me additional 90 days to submit the document, which was almost end of Nov. So that was good enough time. My wife's PCC came in around 1st week of Oct and I submitted the same on the same week.
CIC is aware of the delays at FBI and they are considerate of that. But you need to ask for realistic time; 2-3 months and they would consider. If you ask for 4-5 months, they'll decline. So, be clear and specific also provide them evidences that you are chasing, communicating with FBI for the document.
So for you, you submit your application now with an LoE, and when CIC sends an ADR (usually in 3-4 weeks after AoR), you can evaluate your situation. When they send ADR, they usually give you 90 days, so if you need more time, raise a CSE and ask for additional time. Hope this is clear and helps