I was getting massive underextrusions from a bowden tube purchased from gr5 store.
And I found this:
Fluorotherm F015092 - 5 Fractional PTFE Tubing, 1/8" ID x 1/4" OD, 5' Length, Semi-Transparent
It's long enough to make two and works like a charm. The OD fits the white clips perfectly, and I am able to use 3mm (as opposed to 2.85mm) filaments as well.
The only downside is that it's much less transparent, making light-colored filaments hard to see through.