Another thing- What is the actual OAL of your bullets? Mine measure 1.627" on average, which is shorter than advertised. That'll help with stability, as well...
Using the stability calculator, it looks like you may be at an SG of around 1.53 in 40 degree weather at your altitude, assuming you have a true 9" twist, using the OAL of my bullets. If so, you'd be barely reaching the full BC of the bullet, which jives with your observations. Just don't take the rifle down to sea level in the middle of winter
