Whole lotta pressure... what gives?

I just saw this but I wouldn't assume that a closed overflow tank would help out much. Once enough fluid entered the tank, the air pressure above the liquid would equal the cap pressure and pressure in the radiator (13 lbs) and any more pressure in the system would cause the cap to fail and steam. I have honestly never heard of a totally 'closed' system. Even new cars I have seen have a hole at the top for venting and overflow.
I am very surprised that the tank from Summit did not come with a vent already installed. I think it's important to have atmospheric pressure in the tank.
I think you've solved your problem.

I am just finding this thread today otherwise I would have spoken up sooner. All the newer cars that I have seen that have coolant overflow tanks are vented to the outside. A system that gets hot will expand, without a vent the pressure will continue to build until somethings pops and relieves the pressure. Your pop bottle worked because it was not sealed and could vent the pressure. Your problem started when you ran the coolant overflow hose to your closed catch can.

But it seems to be this is the conclusion you came to anyway albeit with a bit of unnecessary worry