Does the thinkpad not come with a tool for recalibrating the battery? Something that discharges then recharges it while monitoring ?
As far as replacement batteries, if it's not OEM, the only other place I really check out is
www.apc.com You might pay a little more, but at least you have quality and good warranty to fall back on.
I'm not too sure what the lifespan is on newer batteries, but we always used to replace them after 3-4 years of use.