And just how much better are those 6.5 ballistics at 600 yards than the .270 Win?
Great question, here's what I found:
Using JBMBallistics (59*, 50% humidity, 1000Ft elevation, 29.92, and a 100 yard zero)
76.5" Drop, 21.2" Drift, 2015.1 FPS, 1280lbs
161.1" drop, 40.1" Drift, 1772.4 FPS, 990.4
73.1" Drop, 25" Drift, 1966 FPS, 1159lbs
157.9" Drop, 47.9" Drift, 1678.7 FPS, 844lbs
In an attempt to compare apples to apples, I used 142 GR SMK (.595BC) @ 2850FPS at muzzle and 135 GR SMK (.488BC) @ 2995 FPS @ muzzle. I picked these velocities by going to federal premium and they had the 260 Remington firing the 142 GR at 2750. Since the .30-06 typically shoots about 100 fps faster than the .308, I added 100fps. The 270 had 130 grain bullets firing at 3050 and the 150 Grain firing at 2830. If you do the math, assuming a constant change in weight causes a constant change in velocity, you get the 2995FPS for a 135 grain bullet.
A known distance is a lot easier to adjust for the elevation than shooting through an unknown wind and at unknown distances the drop is more similiar than the wind drift. AT 600 yards, you have almost 4" less drift and at 800 yards you have almost 8" left drift. As I mentioned, the 270 is a great round and I own one so I'm not saying its a bad choice. I'm just saying the less wind drift is what I considered the asset and if I was choosing a cartridge for the OP's purposes, I would go with the 6.5-06 over the .270. I agree with Taylorce1 that it's not a huge difference and I'll even admit the actual numbers were closer than I expected for drop.
On a positive note, I didn't even know the SMK was made in 270 so I'm pretty excited about that! I'll probably load some of those up.