I've both the 6-cell battery and the bay battery. By completely draining both batteries untill the computer shuts off, I'm now back to 99 % capacity for both batteries (quite weird, don't know why...)
This means I have a total battery capacity of 67Wt. When disabling wireless and turning the display brightness to lowest, idle power draw is around 7W, in theory enough for 9½ hours life. I can bring the computer without AC adapter to the university, and with some lectures and lunch, keep it running from 9-17, sometimes with wireless and okay brightness.
If the price of the bay battery was not so absurd, I'd recommend it right away. I bought the laptop second hand, and had it included, so I didn't have to swallow the cost. My friend has an X300 as well, with only the 6-cell battery. If you find somewhere they can be bought inexpensively, please give a notice
My battery is 16 months old. I'd recommend you try creating a profile in battery manager that doesn't hibernate or put the system to sleep when battery level reaches x %. Then let the computer run out of power, so that it turns itself off. I think that's partially what caused my battery capacity to increase from 82% to 97% and now to 99 %.