I've been told that more than half of investments are pre-notified before closing, but that's a figure you should try to obtain from the investment review branch because they track that information. It's not available publicly, but they would have that information if you require it.
The other thing to note is that Canada receives notice of about a thousand investments every year, and only a handful of those investments are ultimately reviewed on national security grounds, which shows that the vast majority of foreign direct investments into Canada are not subject to a national security review, and only a handful are. This suggests to me that either investors are being prudent about the types of investments they are making or that the government has accurately identified the types of investments that raise national security issues.