regear, regear, regear..... with the stock gearing it takes alot out your set-up to get the truck up and rolling, try smaller pinions until you find the "sweeet spot" and then with your set-up you will be maximized, unless you want to put your pede' on a diet, removing as much weight as possible.....
Li-poly packs won't give you more power, 7 volts is 7 volts it doesn't matter how it gets there.... however they will make your truck faster because they are lighter and the higher capasity will give you longer runtimes....
Without reducing weight and maximizing your gearing you will need to upgrade your electronics and use a faster motor to go faster top-end......