I evaluate and utilize all types of batteries for my work. Here is a very informative site that I have found and utilize a lot.
:link: I should mention that, IMHO this site is overly negative on lead acid and biased towards the other battery technologies that the author’s company provide charging products for. However, most of the information is very good.
In comparison to lead acid batteries, Li-ion has about 3 times the energy density of most lead acid batteries. However, since the power curve of lead acid drops off with discharge, and the life is adversely affected by depth of discharge (DOD), lead acid batteries are generally only discharged to 50% depth of discharged so the effective Li-ion energy density is almost 6 times higher. So if you have a 50 lbs lead acid battery you might get the same amount of power out of a 8.3 lb Li-ion battery.
However; you should also consider peak load current, over charge tolerance, operating temperature range, self discharge rate and cost.
Overcharge tolerance: In the link I provided above, you will see that they lead acid battery over change tolerance as “high” and Li-ion as “very low”. If you are using maintenance free valve regulated lead acid (VRLA) batteries, such as AGM (Optima is an AGM), then I would change that to moderate. Li-ion however has very low overcharge tolerance especially in high ambient temperatures. I THINK you need a special Li-ion compatible voltage regulator with battery temperature feedback if you're going to switch to Li-ion.
Operating temperature: The link I provided shows the operating temperature range for lead acid and Li-ion as -20 to +60 degrees C which equates to -4 degrees F to 140 degrees F. In the case of lead acid, my experience is that many lead acid batteries will work at -40 degrees F (-40 degrees C). My experience is Li-ion is their output drops of dramatically below 0 degree F or -32 degrees C. Also Li-ion are very unforgiving in high temperatures.
Cost: The link I provided above shows a 4:1 difference is cost.
For a race car where cost is irrelevant and weight is king Li-ion might be a good choice. For an off road vehicle where weight is not that significant but reliability and dependability are king, I think it might not be a good choice, especially in very hot or very cold conditions.
The preceeding comments are only the opinions of the author. I have not interest in or allifiation with any battery manufacturer or charger producer.