At low temperatures the performance suffers but within reason you get a little longer life. Very low temperatures pose many problems with freezing on part discharge and low performance on heavy loads.

Here in the UK the normal temperatures cause little problems as we don't have the extremes. Temperatures from 10 to 20 C seem to be no problem.

In certain military applications they heat the battery boxes in low temperatures to get the things to perform but I don't think life is the main consideration for this.

Batteries die if you use them, they die if you don't and they die a lot quicker above 25c but you can't win. AGM seem a bit less troubled by sulphation on part discharge but you pay a price for them and although they tend to last longer under some conditions I doubt that that they are more cost effective. They certainly won't stand the abuse of the old wet Plante batteries but they don't need the constant maintenance either.

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.  

Have had issues in posting a comment for last week or so, talking with Dan F about it.

Did more research and found out the the optimum temperature range for battery bank is between 65 - 75 degrees farenheit.