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Did further testing today, and I'm fairly pleased.
First tried cranking the car through jumper cables with a full battery. No good, contact resistance too high.
30 minutes with blower, high beams, and ignition on: 54A@13V, 48A@11V ish, so I drew around 26Ah of charge from the battery. At this point, the voltage under load was around 10.3V.
I then turned off the blower and high beams, leaving just ignition and low beams on, measured current was around 23A. Left it on for 15 minutes, drawing another ~5-6Ah.
Then I adjusted the jumper cable clamps a bit to get a better connection and attempted to crank the engine. To my surprise, it actually managed to get through one or two compression cycles! I turned all the electrical loads back on to get 45-50A of current again, and found around 2V of drop at the car's battery terminals, so it's no surprise I wasn't able to crank the engine, but I have no doubt the battery would be able to do it with a better electrical connection.
That said, after a few failed cranking attempts, the battery voltage was dropping very quickly, so that was basically all it had left. Interestingly, the open circuit voltage actually recovered to 10.8V within a few minutes, so maybe it actually has a little more juice left. With ignition switched on but nothing else, it reads 10.45V at my multimeter. I actually feel pretty confident that the engine would crank with this voltage, though maybe not at -20 degrees.
I think I'm comfortable believing the usable capacity of this battery is 32Ah, and will remain over 30Ah for probably a few years of use, so my little gamble hasn't been all that stupid. Now to obtain a European gas guzzler to put this battery in...
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