The amp hour rating is how long the battery can sustain a certain level of current drain before running flat, so a 2.5AH battery is rated to give 2.5 amps for an hour, or 1.25 amps for two hours.

If you have a charging system, then it's basically moot, as that will charge the battery in any case. Unless you spend a lot of time at lights idling (where the charging system is least efficient) as this is when the battery takes up the slack. The other time it matters is when you have a start motor to drive, so not applicable to us.

The Shorai battery is about the cheapest way to lop 2.5kg from your NSR AFAIK?