"I got 2000mAh ones vs the Therm-ic 1400mAh. I saw up to 2900mAh ones which would more than double the capacity of the original ones!"
A free PSA: these numbers are completely bogus. They all are, on every battery.
All Ni-MH batteries are the same, differing a percent here or there in construction and materials. Cut any one of them open and the guts are the same. Since battery capacity is simply a function of electrolyte volume (the limning factor), all AA Ni-MH will have roughly the same capacity, again, within a few %.
So why the 100% rating difference between one battery and the next? Because they lie. No really, they just make up the numbers. Ask anyone in the industry.
There's no testing and no standards, so when the cheap brands started listing ridiculous numbers on their packages to make them sound better, the bigger companies had a choice, demand testing for everyone, or simply increase the numbers of their own packaging. The later was cheaper, and the numbers have been climbing to this day.
Don't get me wrong, a new battery will absolutely have more capacity than old ones. But a new 1500 mAh vs a new 2000 mAh is likely to be identical.