We are in the process of converting the 14 year old Gel batteries on our new Soggy Paws (a catamaran) to LifePO4 Lithiums. We chose a DIY approach, and ordered 3.2v cells from RJ Lithium in China, a total of 8 cells which results in a 540 Ah battery bank, that takes up about half the size and weight of our previous Gel batteries.
RJ Lithium LifePO4 CellsWe have just put them in and are still finishing the install. But before installing, we did some capacity testing, using a precision load, one cell at a time. Here is the result for one cell, we took a full cell that was resting at 3.3v (13.2v in a 4 cell battery) and put a constant 35 amp load on it, and it took 7 hours for the voltage to drop below 3.15v (12.6v), and we got a measured 250Ah (from a 270Ah cell). It's pretty amazing!

You do need to make some adjustments to your battery charging and monitoring methods as using standard lead acid chargers/charge profiles will kill your lithium batteries (and/or your alternator).
We'll let you know how we like it after we've cruised with them for a year.
A good place to start learning about converting to LifePO4 batteries is here:
https://marinehowto.com/lifepo4-batteries-on-boats/and this one:
http://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/Sherry