It will defo do something, what is a hard question though.
If you fit one 3mm thicker this is a massive amount in stiffness terms, likely to promote odd handling when pushed, unless you fit the suitable rear one as well, as one end stiffer than the other.
I did the opposite on a fwd car, put on a much stiffer rear arb, the understeer was terrible I have the suitable front one to fit when weather allows.
tabetha