I'm running the 6.5-20x40 w/30mm tube on my go to hunting rig, the 40mm objective allows the rifle to actually fit in my backpack scabbard & still allows a good amount of light to be transmitted during low light situations (when the critters tend to start moving again). The 50mm objective WILL transmit more light, how much I am not certain, I have yet to feel handicapped with the 40mm. The CDS system has merit as long as you always hunt in the same place at the same time. All variables have to be the same for your point of impact to be the same. Elevation, Barometric pressure, temperature etc will all effect your bullet flight, the CDS (as well as similar systems) is set for specific set of parameters you provide to Leupold. If they are set for the mountains & you head to TX for a long range hog hunt, you could very well find yourself undershooting your quarry & so on...
I was running the MK4 with the TMR & M1 dials... I just traded it off, as I just couldnt get over the dial MOA hold in MILS thing. The TMR reticle is awesome & I totally favor it in my MK4 M5-A which is setup in MIL/MIL. 4.5-14 will absolutely get you to 1k yds, however my favortism tends to lean to the higher magnification range. I'm running/have ran 8.5-25x50 MK4, 8.5-25.50 LR/T, 6.5-20x50MK4 M5-A, 5.5-22x50 NXS, 6-18x40ao VX-II, 4.5-14.50 Zeiss.
With a higher magnification you can always dial down, you can't always dial up with a lesser magnification optic. Keep in mind, a 4.5-14 MK4 will have more elevation travel than a 8.5-25 MK4 (IIRC).