You sure? What is the AH on it?
According to this post from the other Tesla Forum it's a smaller 35Ah battery.
Lithium Ion Batteries are typically 70% lighter than the same size lead battery.
Lithium Ion Batteries can last 3000 - 5000 cycles whereas lead batteries last 300 - 500 cycles.
I agree that replacing with a Li-ion cell would be a good idea based on the high OEM failure rate.
As an example, here is a 12V, 34AH, AGM battery that is 26 lbs shipping weight: 12 Volt 34 AH AGM Battery I'm sure that the battery itself is 25 lbs or less. A 70% saving on the weight of this battery would be 17.5 lbs.