Cats are effective mufflers. This would definitely help, but I can 't say how much. Cats are also expensive, so it'd be a costly experiment. It would help with the exhaust smell, though.
I don't see the point of a universal resonator positioned AHEAD of a muffler. A resonator is designed to attenuate and cancel out a specific range of frequencies. If the muffler's tonal effectiveness was a known factor, then yes, you could pair it with a resonator ahead of it as they'd be designed to work together to achieve a specific output sound. Generally, a resonator is placed after the muffler to clean up some undesired frequencies rather than lower the output volume. People often throw resonators in just because they're smaller to fit where something was and it does something. Resonators are to alter the sound, not so much the volume. Sure, if the sound changes enough, we're gonna perceive that as "louder" or "quieter". Straight-through mufflers, such as your Borla and that Dynomax I linked to primarily adjust volume. Yes, they absorb sound frequencies so they will produce a certain sound signature that is clearly different from a chambered or baffled muffler. What I like about the straight-through design is that it's quite effective at lower RPM and volumes, giving a [relatively] quiet idle and cruising sound. But, when the gases really start flowing, they get proportionately aggressive. Best of both worlds. They also always flow about as best as you could possibly ask for. Chambered and baffled mufflers manipulate the waves in a manner to "color" the sound. Most are still just a "can", so they tend to be louder at idle and cruising. For obvious reasons, they also don't flow as freely as the straight-through design.
Did you keep your stock exhaust stuff? You could have your original third cat put back in in place of that resonator that likely isn't doing much, anyway. If it works for you, it'd be a cheap solution and reducing the exhaust smell would be a bonus.