Thanks for the clarification Rick and John,

I just wanted to reaffirm that we could still use the 2X from 1000-1200mm with Live View and AF on the LCD screen. Sure, I remember those days with Manual Focus on my Pentax ME Super 35mm, but it had a split focusing screen. Maybe we would eventually need to add one of those aftermarket.

John, I'm a little concerned myself that the 600mm is a bit too heavy for hand-holding for a few hours, and I'm a weightlifter. The 1DX is pretty heavy by itself and then factor in the added 2X and it was noticeably heavier than the MKIV with 1.4X. I'm not concerned at all on the monopod.

To take my own advice I think the 500mm would work for me for sports, travel, and handholding, and then add the 1.4 @700mm and it's nice to know that I could use LV with the 2X @1000mm on a tripod for nesting and stationary birds or use the MKIV.