My wife and I have the Keen Growlers as well and just used them for several days of snowshoeing in 7F with windchill down to -11F in Northern Vermont. I much prefer winter shoes that are rated to -25F, rather than shoes rated to -40, because -25F shoes like the Growlers will work well for you in late fall and early spring, and when it's very cold, you can extend their warmth with 1) an overboot like the Neos Trekker or one of the Forty Below overboots http://www.40below.com/products_Results_overboots.php?S_CategoryName=overboots ; and/or 2) 3-4 mm Neoprene booties like the Crescent Moon, which go over the Growlers. You can wear the booties alone, or if you get big enough overboots, you can wear the booties over the boots inside the overboots.
With this setup, we can use the Growlers late fall to early spring.
For the recent conditions we had in Vermont (which included 18 in. snow in about a day), the Neos Trekker over just the Growlers was fine.
However, we exchanged our Neos Trekkers and are picking up Forty Below overboots, because the Neos Trekkers don't stretch and didn't have a lot of height allowance in the front, and in order to work with neoprene booties underneath, we would have had to size way up (and even then it could have been tight). I have a size 11 foot, typically wear a size 11.5 hiking boot and wear a 12 in the Keen Growlers - I needed an XL Trekker rated size 11-13, and even then there was just enough room to squeeze the boot in. I should note the Trekker was meant for warmer weather use, such as fly fishing and river running.