ok, here are a couple thoughts, but thoughts usually aren't just pennies in cost.
if you are way too heavy, try going to lithium polymer packs. the packs that usually come with these things are nickel cadnium packs (nicads) which are heavy but can put out a lot of amps. they also dont have much capacity so your flight time will be limited. lithium polymers are newer technology. they come in flat cells that are 3.2 volts per cell as opposed to 1.2 meaning that a two cell pack would make a motor pack. a speed 400 will likely draw 10-12 amps, so you want a battery that will put out that much current. something like a 2000 mah pack would do nicely. now for these batteries you can not use the charger they gave you (unless it says i can charge lithium polymers or lithium ions) because it will continue to charge until the cell explodes (not leaks, i mean explodes violantly) which is not good.(duh) so li-poly batteries (lithium polymers) may be the answer here. go to your lhs (local hobby shop) and ask about them. they are sure to know, and they can probably reccomend some compatible equipment to use as well and will tell you all you need to know. that should knock off several ounces right there and should give you significantly longer flights. you speed 400 is probably fine