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For lead most EV users like to use a 50% dod to maintain decent cycle life (800 cycles but thats usually optimistic). LiFePo4 are rated at usually 2-3000 cycles at 80% dod and 3-4000 at 70% (probably an optimistic number as well), but there isn't too much info out there on actual long term use yet.
The resting voltage of a single lithium cell is ~3.35V. That gives you a starting voltage of 13.4V vs 12.7 with lead. Its not huge, but it will keep your headlights and other things a bit closer to the target voltage, although there will still be voltage sag (more or less depending on temperature and load, but still probably less than lead acid). You will not be anywhere near 15V though. The max charge voltage is 3.65V/cell for most lifepo4 cells (if you want to maintain good battery life), but once they're off the charger they settle around 3.35V.
My question to you is why even keep the lead acid starter battery? The lithium batteries can easily start your car. If you need to go on a long trip there is no reason why the alternator can't be switched on to maintain them. For the rest of your driving you can just run without the alternator and charge them when needed.
I also wouldn't buy those cells. I'd buy some individual thundersky or calb cells. Those don't even have a name brand and the TS and CALB cells are rated for more cycles and higher C rates. I have cells very similar to those in my Prius PHEV kit (40ah mottcell cells), so I'm quite familiar with using them.
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