The foam clunk is there to try and prevent bubbles from entering the fuel lines. The pump has a small piston and small stroke so any air in the system in the pump or downstream from it can upset it's ability to generate full desired pressure and/or draw more fuel from the tank. Just think of air trapped in your car brake system, the fuel system gets a bit spongey untill the bubbles pass through.

A bubbleless tank will perform the job provided you get all the air out.

The pump will self clear bubbles 99.99% of the time, it has to as I break my fuel lines between the pump and tank to fill it which can leave an inch of air in the line, but I wouldn't want to be a full throttle when that inch long bubble hits the pump, just in case of a lean misfire.

I have no experience with the non-cdi version, but rumour has it they need to be run fairly rich?

Cheers