In case it matters to anyone, maybe someone that was planning on doing something similar...
Anyway, what I ended up doing was buying a 1DmkIV (out of the Classifieds here) and a 400mm f/3.8 L IS USM (also from the classifieds here), so that is where I am at at this point.
I just got back from a short 3-day trip (800 miles) from Texas to Southwestern Oklahoma to the Wichita Wildlife Mountain Refuge
, I LOVED that area, I'm going back, real soon... (I highly recommend that place, when it is a little cooler, like in the Fall!)
I used the 400mm lens and a Kenko brand 1.4 extender to help me along. I got really good results with it, that is what I wanted, so my purchase (to me) was a good one. I do plan on getting a Canon brand 1.4 II extender soon, I don't need the options that a 1.4 III has, at least not that I am aware of.
The focus speed was fine, better than fine, I couldn't tell I had an extender on actually. I took only a handful of shots without the extender, so I can't say what would've happened had I not had it on. (I'm sure it'd be good though!) The ONLY time I had any problem at all was when I was at Possum Kingdom Lake (In West Texas) and I had a buck (in velvet) in my sights and I couldn't get him all in the there.
I was actually asking about BiF when I posted, and to be honest, there weren't many shots I had a chance to do that with. This 400mm f/2.8 is HEAVY, (12lbs not counting the 1DIV or the 1.4 X) and I shot 99.9% of the time hand-held, 'just because', really, I didn't want to hassle of me setting it up every time I stopped. I mostly shot from my SUV, I had a piece of closed cell foam that I slit to fit my window, it worked PERFECT!
I had planned on trying out the Kenko 2x extender also, but I had such a full schedule, I didn't even get it out once. I had also planned on stacking them, 'just to see', maybe next time. 
The only time I used my monopod was when I was videoing the prairie dogs for my wife so she could see them, and that was using my 400mm/1.4x on my 5DII.
So, really, I got exactly what I was needing and wanting, the only drawback to this set-up is the weight, of course, I have a monopod and a gimbal mount for when I do get to shoot BiF, or hiking any distance.
I ain't going to lie, I trembled and sweated when I held it any length of time at all, and I know it's not to be hand-held, but I had to in the way I was traveling. Some of my shots showed that I was having trouble keeping the camera and lens steady, so my focus was occasionally off, but I'm still learning it too, hopefully I'll get better soon.
What I should have done was have the 1DIV with the 1.4 extender on my 400mm f/2.8 lens, my 100-400mm L on my 5DII, and brought a 3rd camera, my T2i with my 100mm Macro lens, that would've been perfect! I say that because I like macro too, and the insects were everywhere out there. I had a 14mm wide-angle Rokinon lens for landscapes on the 5DII most of the time, and sometimes switched it out for my Canon 28-105mm.
Mostly, I took landscape shots, but some mammals, (Bison, Longhorn, and prairie dogs) and some birds, mostly sitting, not many flying, and some insects.
All I know is, I had the best time I have had in a long time, saw a LOT of cool stuff, and RELAXED, really a good time, cameras or not. (O.K., that last part is more of a lie, but it sounded good!) heheh
What was strange to me, I didn't ever mount my 100-400mm L lens on any camera, it stayed in my bag, that is a first, it never, EVER left my main camera before.
Thank you to all that helped me understand my decision, and helped me think of what I might need, you never know until you ask... 
Randy