Hello!
BackPacker: Anyone knows the capacity and self-discharge numbers for those?
They vary widely between manufacturers and battery models. As usual: The more expensive they are, the higher the capacity and shelf life. I think your 3000 mAh figure is a typical value. If you look closely, all alkaline batteries (at least those of major brands) have an expiry date printed of them, normally between five and ten years in the future!
Like you, I always have a pack of new alkalines in my flying bag as backup, but I find four of them sufficent (that cost together as much as one Eneloop cell!). I replaced the last pack recently after carrying with me unopened for over five years... and they still go strong inside some toy of my son.
It is important however, not to leave alkaline batteries inside devices like GPS receivers, because they always draw a little current even when turned off (to power their internal clocks etc.).
Greetings, Max
NB: GPS is here to stay. Learn to use it properly, and the flying experience will gain a new quality: Simply enjoying to be in mid-air, without having to worry about getting lost or infringing airspace. And a spare GPS unit in your pocket will give you a lot of extra peace of mind.