By flashing do you mean rooting and loading a different rom? It depends on what you load, but it won't necessarily improve the performance. I would only do it if there is a specific feature that you want on the phone that currently isn't available.

I have 2 SIII non activated phones, one Metro POS & the other Sprint. Is it worth flashing them. Not sure if I'm going to activate them. If I did, it would probably be with Straight Talk.