In a 270 Win a 58.5 gr load of RL-22 behind a 140 class bullet droped 130 FPS from 60* temps to teens. It turns out that the Chrony was/is temperature sensitive way more than the powder is.
Thus I don't worry so much about the temp drop but I do compensate a bit for it. Its one of those last minute "fudge" things. If I 'feel' its needed. Works for me
Considering the bullet you are using, I'd stick with the 600 yard limit, as indicated by your 300 yd worse condition comment. The idea is to slay the elk.;)
For WIW I switched from 150 Ballistic Tips to 140 NABs in my 270 AM because of the better bullet construction. Both have the same BC but I get another 100 FPS out of the 140.
Both bullets shoot exceptionally well in the rifle.
My thinkin' is that I'll get just a schosie bit better chance of complete penetration w/the NAB. However, I shot a Muley buck w/a 130 BT that went through 30" of deer with nearly perfect performance.
Also, I want to attempt one of those high shoulder shots one of these times, especially in one particular spot that I hunt. DRT would mean way less work.
BTW, how far are you from Dirk Parkinson's farm up at the end of Hog Hollow road?
It's great chuck shooting across the old Teton Dam spot. Though there seem to be only a few chucks.