Catalytic converter failed or something else?

[SoundsDigital]

Had my V50 about 6 weeks now. Bought it with the check engine light on. It was flagging an egr fault. Replaced the egr and it's been fine for a few weeks but now started throwing a P0420.

Taken a reading with Vida of the two O2 sensors and both look almost identical.
I'm guessing it's probably the cat, which after a new egr valve and a/c condenser is making what was a cheap car pretty expensive.

If it was an O2 sensor, I am assuming the two sensors would give different or no readings. So in my mind this is looking pretty likely to be the cat. Before shelling out more money on it though I would appreciate other's opinions and open to a quick fix.

I did put a bottle of cataclean in the tank and had no faults until I refilled the tank. I've seen comments about using premium fuel to overcome a P0420. Any truth in these claims?