Or, as you note, the Galaxy's expansion feature (a zippered gusset along the side allows the user to add up to eight inches of girth) gives you room for an extra insulation layer. Mountain Hardwear's Down Upgradeessentially a light down bag that can be inserted or zipped inside another bagsells for $180. By itself it's rated to 40 degrees. What it would add to the Galaxy's temperature rating is a little hard to estimate, but I think it's safe to say the Galaxy and the Down Upgrade would work well down to minus five.
Good enough for a Denali summit attempt? Probably not. For one thing, the combined bags will weigh nearly six pounds, and you'll be carrying plenty already. For another, a bag rated to minus 20 must be considered the minimum up there. I think you're going to have to review your trip budget and consider purchasing a bag just for this trip. But you needn't spend a tonL.L. Bean makes a good quality, minus 20 down-filled bag that sells for about $350. Or, if you want to splurge, get a Western Mountaineering Big Horn Super MF (minus 20, $530). Both weigh less than four pounds, a load difference for which you'll be grateful. And, that vapor barrier liner will work well with both, adding to the temperature range if it gets REALLY cold.
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