Done properly, you should not see any difference.

If you put a 1/8" ply sheet over the existing one, merely decrement the firewall to prop driver distance by 1/8" as well.

As DaRock has pointed out, you should also add tri-stock to the area behind the firewall along the areas the firewall joins the fuselage ply.

The idea is to distribute any torsion or stress over a greater area.

While you are in there also consider adding some ply or hardwood stock behind and against the existing firewall for good measure.