It is already in thrds and I have tried it both with 1/3rd front and 1/3rd back and didn't like either for different reasons which is why I'm thinking 50/50 may be better. I've seen other people do like you and add weight under the rear foam but rather than adding weight wouldn't it just be better to find the right battery placement in the first place?
As for rear toe 4.5 degrees sounds quite extreme...I don't think I've ever seen a car run that much. My current setup is whatever stock is in the back and grip was fine so long as I had 2/3rds front and 1/3rd back...but with the heavier battery the truck didn't jump very well. Most of our track is carpet remnants so grip is good there...it's just in the few turns that are dirt is where the traction suffers.
I've tried the plastic caps a few times in the past as well and I've never liked them. I figured I'd give them another try on this truck and find I still don't like them. I wish they would make aluminum ones with the bleed hole but they don't.