It's probably heresy to a lot of people running a short pack, but when Masami says "More traction, more power", I tend to listen.
I saw this video about a year ago and I went out and tried the short pack versus the full sized pack on a weekend, and the full sized pack was better. Every time I have tried it, with one exception, it was better. I'm sure that on a hot lap, on a perfect track, the short pack may come up on top, but over a run the big pack has always been better for me. We are racing 2wd pan cars on rubber tires, in many cases on asphalt, so traction is at a premium. Any time you can get grip into the car is usually helpful, as then you can then dial in more steering, instead of trying to tame a car that wants to do donuts.
It's not like I am against light cars at all. When F1 foam was more popular, and 540 brush motors were the norm, I ran mini revo 1800 mAh packs, which weighed about 100g less than any other normal lipo. It worked with the lower amp draw motors and the high grip of foams. Now, not so much..
In my experience, the full size pack is easier to drive, and sticks the car a lot more. You can leverage the traction into a car that steers more, and leaves the corner faster since you don't have to baby the throttle. Mistakes are not as costly..I know, I make enough of them....
The one situation where the short pack was better was on the infamous Pardus tires. The break away characteristics of the rears was such that with the big lipo, the car wanted to keep turning , over rotating. The added weight toward the rear of the car was swinging the back of the car around. As soon as I went to a short lipo mounted closer to the front of the car, this tendency went away and the car was just the way I wanted it. On other tires, like Pits, this is not a problem.
Maybe you don't agree with me, but just try the two different packs back to back, and see what you think. Pretty soon, you may be trying a steel axle, too, but that's another post.