Hi alone6288,
your plasma will be very far from any equilibrium, so rules for equilibria like Le Chatelier and Kc don't apply.
At identical pressure and temperature, 1L of N2 and H2 mixture can't make 0.7L of NH3, because the product has twice as many atoms per mole.
My intuition, and nothing more, tells that a plasma favours small molecules, so extracting the NH3 as soon as it's produced seems necessary.