If the new battery physically fits in the space available it will weigh less. Its just a case of finding the most capacity for a pack that will fit the same physical space, and if there is a choice, getting the highest capacity. Since claims of battery specs change rapidly, the boat manufacturers can't allow for that in their printed instructions which will have to last a long time. So just look for the biggest mAH figure that will fit in the space available.
I would be very wary about using LiPo - the semiconductor specs that I have read for the ICs used in servos usually quote a top voltage of 7.5 volts. Hooking them to a source that can charge to more than that and deliver a lot of current is likely to reduce their life expectancy. I don't know what LiFe charge up to, but it should remain in the relatively safe zone. Running servos on a higher voltage causes them to give more power - the downside is that if they have to work harder, like if a rudder catches on something, the higher spec battery might be able to deliver enough power for the servo to damage itself.