Would you downplay a GHG that had a radiative forcing value of 20% as being insufficient?

Question

insignificant (sp) either or, works.

Answer 1

20% of what?  Insufficient for what?

Your question is just a tad bit vague.

Answer 2

No.  20% is certainly "significant".

But equally important is whether or not it's increasing steadily like CO2.  If it was constant, like water vapor, it wouldn't be causing an increase in temperature.