kiwi3006 provided a link to a LRH thread, which contained a link to this thread at the Sniper's Hide. Thanks kiwi.
Spindrift Calculation?? - Sniper's Hide Forums
Now I don't advise anyone to read that whole thread unless you're really into this stuff. It's a very, very long thread and it took me about 2-3 hours of time to digest the entire thread. Summarizing, the jist of it is that spindrift is for real, and with my rifles and bullets, I could expect to see about 10" (~1 moa) of left-to-right spindrift (based on right hand twist rifling) at 1000 yds. So spindrift could indeed explain most of the left-to-right drift I'm experiencing with my 300 Win Mag.
Spindrift doesn't account for the excessive left-to-right drift I'm experiencing with the 7mm Rem Mag. I'm getting about twice the drift attributed to spindrift with this setup. So I'm going to take a good close look at the scope set-up on my 7mm and make sure I haven't mounted the scope with a slight counter-clockwise rotation from vertical. If the scope were canted counterclockwise, then when I leveled the crosshairs, the bore would send the bullet downrange with a slight right drift. That's currently my best explanation.
Any other thoughts, I'm all ears. Other than that, I'll let you know how this pans out.