pack...I have seen a couple other people running the juststock in 1/12th at our local track....though with reciever packs....probably never tried without....I even run mine with the factory fan removed.....I feel that with such light weight and efficient cars compared to a touring car there really isn't a lot of load put onto the motors which in turn keeps the load off the ESC....I plan on being up at the track today about noon for an afternoon of practice.....the new layout is kind of tough and only living 10 minutes away makes it nice.
your question on the hump pack I really don't know beside I kow there is an extra pain of keeping them charged...One of our really fast 1/12 guys last race had to bow out of the main when his went to start the race and his pack was dead....forgot to charge......If I were you I would try another jusstock or spend the coin on a 1s ESC so not to have the pain of the hump pack.