Ok... I know this might be a little bit of an odd selection, however, have you considered a 308? Lots of good tactical setups for the 308 in a 20" barrel setup... Many threaded as well, so you could toss on a brake or suppressor if you so desired.
Now I know this is out in left field as you re looking at throwing lighter pills faster... But i thought I'd throw out the opposite side... Heavy pills slower!
If you sent Berger 215gr Hybrids out of a 20" barrel of a 308 @ 2400 fps, you'll get to 900 yards with 1008 ft/lbs of energy (10 twist) and they would still be running at 1453 fps upon impact. If you run them at 2250 fps... Then you would be getting 1101 ft/lbs @ 700 yards and an arrival velocity of 1519 fps.
2155 fps from the muzzle would put you exactly at 1000 ft/lbs @700 yards and still running at a velocity of 1447 fps when they arrived.
Now if you went with a 30-06... You could generate a little more speed.
That's very doable. Sort of big pills to throw... But extremely effective as well. The 215 Hybrids have been shown to be very effective on big game (see Broz posts) and they have a whooping .696 BC. the 308 has great barrel life compared to some of the other options, has a proven record of accuracy out of 20" barrel configurations, is easy to find factory as well as reloading brass & bullets, and you can choose to shoot light pills when you so desire. I honestly think with the advent of the 215 hybrids, new life has been breathed back into the 308, as well as the 30-06,
Something to consider that nobody else has mentioned... which is why I thought I'd throw it out as an option!
Good luck choosing and shooting once you do! gun)