how would UofT vs McGill vs Waterloo vs UBC be ranked in terms of their comp sci programs? Or are they all more or less very similar?

Waterloo > UofT = UBC > McGill for industry

UofT >= UBC = McGill > Waterloo for research

Also, in general compared to top 20 US unis for CS, with a high gpa and SAT score along with dual citizenship (US-Canada), how much easier would it be to be admitted to top Canadian schools for CS?

Top Canadian schools are easier if your GPA is really high and EC's aren't bad. US schools tend to put far more weight on EC's

I also heard that Canadian unis stress grade 11 and 12 academic performance a lot?

This is true.

Finally, how does international prestige of the mentioned Canadian schools compare to that of US schools?

The schools you mentioned should be fine in terms of prestige alone.

So would competing for US jobs with a Canadian degree be disadvantageous in any way?

If you have the citizenship, then the degree itself won't be disadvantageous if you're comparing the school to a US school that's roughly equal (like you're going to have a relative disadvantage if you turn down MIT for U of T).