A "normal" battery needs to be topped up with an Optimate (to avoid permanent sulphating damage), so topping up an LiFePo to handle the parasitic drain should not be an issue. But the latter can be run dead flat and recharged without damage. A lead acid will be ruined when that happens.
A "normal" Pb battery has a relatively high AH figure because its not possible to fully discharge the battery without damage. It may take 14 to 16AH to fully charge but it will never deliver that much before the voltage drops dangerously low. If its also powering a starter motor the usable AH capacity falls further. "Deep cycle" batteries used in boat and caravans will deliver more % capacity but they are unlikely to have much engine starting punch.
For use as a starter battery the main figure is cranking amps. The next figure is how many start attempts it will deliver when cold. A Pb needs enough excess capacity to cope with the loss of chemical efficiency at low temperatures to deliver that first punch and hopefully start the engine. After that its shot until warmed up. The LiFePo needs enough capacity to heat itself and then it will start the engine. Neither need enough to do anything above and beyond that. Worrying about the AH Pb equivalent is like comparing apples and pears.
I've got an Odyssey 535 which came to me 2nd hand. Its great never even faltered even when the bike had been out in the frost all night, but very heavy and fully fills the battery box. Its almost certainly way over spec for the job. Had that not turned up, I'd have spent the extra on a Ballistic and put it under the petrol tank. 8 cell is probably big enough but 12 cell would be sure.
