When a 'balancing' thread comes up you'll sometimes see suggestions to relocate the firewall or wing saddle to avoid adding any dead weight to a model.
While I think moving battery packs around (we have attached them to the engine mount, or made holes in the tail area) is a good thing to do, I will not go to extreme measures.
I like to use long engine mounting screws and attach lead chunks to the engine mount rather than to the firewall but it's just a personal preference.