No need to speak ill of Athearn, which isn't my goal here, but I finally made up my mind. After speaking with Jérôme, doing some searches and getting feedback here on the blog, I came to the conclusion it was much wiser to keep the Atlas locos and upgrade them.
As stated, we do a lot of slow operation. So far, Atlas GP40-2W performed beautifully. I'm not eager to trade that given fact for better details. At some point, you have to draw the line somewhere and I know a poorly performing loco is worth nothing even if it is the most amazing piece of art. It will end up on a shelf and I'm not a train collector. I can live with Atlas so-so details. In fact, this particular model have quite many qualities.
Yesterday, I had the occasion to get a Tsunami AT-1000 sound decoder suited for this particular model. When I get speakers, I'll perform the decoder installation with Louis. If I'm satisfied, the other loco will be upgraded and both will receive their coat of long due weathering...
Thanks again to John and Greg for their precious input.