So…Canada no longer uses a category based on maximum allowable landing weight?
Well, yes and no. The categories may be based on speed, but then speed is based on aircraft performance, itself a function of aircraft weight and configuration.
I'll use the Q400 as an example because I can at least talk somewhat intelligently about it. Depending on weight and flap configuration, a Q400 could be anywhere from Category B to E. It normally sits nicely in Category C, but I've been Flaps 35 with a reference speed of 114 KIAS quite recently. If I was fully configured and actually had to do a circling approach, I'd normally maneuver 5 to 10 knots faster than VREF if I was really tight for space (Penticton's runway 34 when approaching from the north is a great example of where you may have to do this)
Using 5 knots on a non-gusty day, I get 119 KIAS to maneuver with, and thus I can stay within Category B minimums. On the same aircraft on a gusty day, I'm using a 10-knot correction and now I'm at 124 KIAS and within Category C minimums. But this is almost theoretical. If I actually had to circle in a medium or large aircraft, I wouldn't be using such tight tolerances.
Now, take that same aircraft again but at a slightly higher weight with a flaps issue on a hot day in Calgary and my reference and maneuvering speeds are much more likely to be the higher end of Category D or even into Category E.
So again, while the categories are displayed on speed, it ultimately still boils down to what my landing weight and configuration permit me to fly at.
As for references, these are going right back to basics: CAR 602.01 and CAR 602.07. Not allowed to do anything that jeopardizes people on the ground and not allowed to do anything outside the aircraft flight manual. Going slower than the AFM violates both of those, so if the AFM says that at a certain weight I fly a certain minimum speed, that's what I do.
Finally, remember that VREF is not a maneuvering speed. It's the speed you're going to cross 50' above the runway threshold. You're not going to maneuver the aircraft during a circling approach at VREF, you're going to do that VAPP (or whatever V-speed designation is in the AFM). A VREF 115 knots may be Category B, if you fly the maneuver at 130 knots, then you use the higher category minimums as with circling approaches we're concerned about the radius of the arc that will be flown during the circle-to-land maneuver, and that's based on the speed we fly that arc at, not the speed that will be used when that maneuver is complete.