Does the real-world battery life scale linearly with it's capacity?
No, because it also depends on current draw. If you have a 10 watt hour battery, and you pull 10 watts from it, you will likely get less than 1 hour of time due to losses inside the battery.
If you instead pull 1 watt from it, you'll get closer to 10 hours of usage, because the losses are not as good at lower current draws.
As batteries age the losses become worse. The internal damage that occurs over time due to the charge/discharge cycle causes additional internal cell resistance. So a new battery will give you more of its rated capacity (and use less internally during use) than an old battery. The old battery will give you less of its rated capacity (and consume more internally than the new battery) than a new battery would.
Is this number a reliable indicator of present and future battery life?
It's an estimate, and now that the battery has higher internal losses it will depend much more greatly on how much power you're drawing. If you're using the processor and graphics chipset at capacity, then add wireless, bright screen, etc on top of that you probably won't be able to use the entire measured capacity.
Therefore it's not reliable, as it depends on your actual usage scenario.
If, instead, you've got it playing itunes with the lid closed, it will use a larger percentage of that capacity before it indicates depletion.
What kind of realistic usage am I likely to get from a new battery under Lion?
Unfortunately you're trying to compare Lion's consumption with your previous OS consumption. I can't answer that question, and even if I had the same model and recently changed the battery I have a different use case, so my experience wouldn't necessarily apply to use.
I can tell you that often one battery will degenerate more than the others in multi cell pack, and if discharged under controlled conditions you can get the full capacity of each individual cell, but under heavy discharge the bad cell is a weak link, and the whole battery pack will shut down prior to depleting the other good cells.
This means that there are several good reasons to replace your battery, and the most likely outcome is significantly increased battery life.
However there's no way to guarantee that due to your individual circumstances and usage. The tools you're using are ok, but without removing and testing each cell individually under a few different discharge conditions you can't know fully how the battery will compare to new batteries. So the tools can't be counted on to give you more than a rough estimate.
If I were in your situation, though, I'd expect to get approximately the same life out of a new battery under Lion as I got out of the new battery under the previous OS. The difference might be slightly greater, but still minimal.
Batteries are the one thing in a laptop you shouldn't mess with -- I'm fairly sure they're the only component in your computer that is in a constantly controlled risk of overheating, catching fire or quite literally exploding. When they are not made to the utmost standards of quality, you could be, again quite literally, putting your life at risk. I'm not saying this is the case with you, and fortunately not with me, but if I were unable to afford a new manufacturer-supplied battery to replace a dying one in my laptop, I would rather sell it and buy a cheaper laptop whose battery I could afford.
I agree Apple's markup on batteries -- and quite generally most of the accessories they sell -- is excessive, but batteries and chargers (due to the risk of electric shock) are two things I would never buy from anyone but Apple. If you search around you will find teardowns of original Apple chargers and cheap Chinese knockoffs, and even if you have zero knowledge of electronics, you can tell the Apple chargers are much higher quality than the Chinese knockoffs. This is quite hard to tell with batteries, but do you believe for a moment the lowest bidder won't cut costs exactly as they do with chargers?
And just to stay on topic, yes the battery is quite important. 7.3 V sounds like a two cell Li-ion configuration while 10.95 V would be a three cell Li-ion battery. Using a two-cell battery in place of a three-cell battery or vice-versa is a sure-fire recipe for disaster -- even if the electronics could take the different voltage, over-charging a two cell battery with enough voltage for a three cell battery would immediately damage it. In case of a high quality battery, only the protection circuit would be damaged, but in case of a low-quality knockoff with no protection circuits, the battery would be immediately and irreversibly damaged, with a symptom of either swelling, fire or an explosion.
Best Answer
No. This A1322 model battery fits many MBP 13's, all the way back to 2009 version. Given the number of iterations and countries MBP have been sold, it's due to have different part numbers.
I also have a MBP 13' Early 2011 version. Two months ago I replaced the battery with a A1322 battery I bought online and I haven't had a problem.
Beware that:
If you are going to ifixit yourself, taking out the battery requires a special screwdriver. I got mine as a free addon from the vender.
Download CoconutBattery. It's a free app that gives you information like manufacture date (which I cannot find under System Information) and charge cycles.