I have no experience with the impact but have tuned a few regulated guns. I think a plenum volume should be at least a little under what is needed for maximum FPE.
If the plenum is oversized by a significant amount, hysteresis in the regulator may cause it to be static (stuck) for several shots until the plenum pressure falls enough to activate it. I want every shot to dump enough of the plenum volume to activate the regulator.
With a small enough plenum, the regulator will activate every shot, and hopefully keep the plenum pressure more consistent, shot to shot.
An oversized plenum can certainly work, but, depending on the regulator, may not be an optimal tune. Oversized will give you maximum FPE, if that's what you want, but it may not give best consistency.