I have to concur with what Flux said, but I'll add my 2 cents because I'm using AGM's in a pretty cold climate, with little ill effect. The batteries are in an insulated shed, but when it goes down to -30C for a week it will be -30C in the shed for much of that time. I put the dump load inside the shed for the winter and the inverter may put out a bit of heat under load, but otherwise not a lot of temp control out there.
Every once in a while I do a deep discharge of the AGM cells to see how they're doing, and so far I haven't found much degradation in their condition since I got them. Of course these were discarded and used when I found them - I could never have afforded to buy these things.
Do you have a temperature sensor on the charge control? (I'd be surprised if you didn't, let me put it that way. Everything else you have looks well done.) I made up a chart for reference so that I can tell how high the voltage should go when they're full. With lower temperatures the float voltage must be raised to maintain a "full" charge. But when I read the installer's manual for these batteries, I found that I cannot let them go above 28.2V. This causes conflict with the temperature compensation, because it wants to charge them above the hard limit of 28.2V. I have to turn down the float voltage knob manually when it's below -20C (aproximate temperature I can't remember exactly). If you can't get by with that kind of situation, then yours have to be inside a heated building.
So when it's extremely cold, I have to live with:
a) reduced charge voltage,
b) reduced performance, (both battery chemistry and inverter efficiency) and
c) risk of freezing electrolyte when the charge gets too low.
That combined state of affairs means that the most full they can get is 80%, they deliver only half the Amp-hours for the nameplate state of charge, and I cannot let them discharge below 50% for fear of freezing the cells.
My 800 Ah is whittled down to only 120Ah! Compounding penalties, but it's just a hobby. I'm not trying to survive off this stuff. Good thing it's almost always windy in the winter here! I've only been doing this for 2 years, and I started out with some abused cells, so I can't comment on "long life" direclty.