Ally used to be hard nosed. US bank seems to be willing to deal. Not too much info on GM financial.
When all is said and done, it never hurts to negotiate. The worst they can say is no and you hand them the keys to the car. The real question to ask yourself is whether you want to lease again, given you are willing to buy this car. I'll bet there are plenty of off-lease Volts in good or better condition than yours. Shop around, see what the going rate is, and see if you can find a steal.