Increasing the acid concentration of an acid should increase the rate of production of hydrogen gas by a metal.
Consider the reaction between zinc and hydrochloric acid.
In the lab, zinc granules react fairly slowly with dilute hydrochloric acid, but much faster if the acid is concentrated.
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)
This makes sense. If we increase the concentration of acid, more H⁺ ions are available to attack the surface of the zinc each second.