I agree with Mithrandir if the question is more based on beam stability; the steel plate will surely increase the effective width & increase r,y.
The last time I heard the term "dimensionally stable" was in my old Materials Lab Class in college; this is from memory, but I thought that related more to resistance to environmental / material-matrix effects (temperature & thermal cracking come to mind)...like a ball of cement could crumble under freeze-thaw cycles, but including good angular aggregate will improve its dimensional stability.
Looks like I have to find my old materials text book...