There should be no advantage in moving from a C2 to a 4-bay antenna for CBC (CBLT). That station transmits from the CN Tower and is the strongest on your chart. If you're not getting it reliably, it's due to the location of the receiving antenna. Relocate the antenna in 6-12 inch increments and see if you can find a spot where the signals behave. It would appear that your current location just happens to be in a "dead" spot, usually caused by an unfortunate reflection from something in or near the signal path. It's unlikely that the concrete overhang, presumably steel reinforced, is affecting reception unless the antenna is within a foot or two of it. Since you indicate you face southeast, that would suggest that you should be able to see the CN Tower unless the high-rise across 427 from you happens to be dead in the way (which would explain a lot).
The C2 already has a reflector. That's the screen grid behind the loop element. It serves the same purpose on an antenna as does a mirrored surface does for a directional lighting device (in reverse), that is, to focus and direct incoming signal energy to the reception element (loop).