It may or may not have been a factor in the focusing. I was mainly trying to test the IS...it worked, but only for about 1 in 4 of the shots at such a long exposure. How many stops of IS is 8/10 of a second at 300mm?
Sorry, I mis-read your posts (just drinking my morning coffee :-)) and removed it, but not before you could snatch it for a quote :-p ... you're correct, with exposure compensation used (which you didn't mention at the time) you cannot directly calculate back the ambient from only shutter/aperture/iso, so if it was as you said it might have been darker than -2 LV.
Another question is how hard these barriers are, much more likely the camera's ability to af doesn't stop suddenly af -3.000001 LV, and nobody really tested it so we can only take Canon's word on how large the 5d3/6d gap really is.
My experience with the 6d af using the 100L f2.8(!) lens is that the lack of a precise center cross point shows up more at dim light, it af's alright sooner or later, but the precision seems to be worse than at good light unless you're extra-careful to pick a contrast part. With a f4+ lens you won't have these troubles of course.
Something we can agree on! I have the same experience as you with your 100 f/2.8, in my same darkly lit room, with my 135 f/2 mounted. It absolutely refuses to autofocus with just center point, where it will easily AF with an f/4 lens in the same light. An f/4 lens is how much less light than f/2?
As for the 70-300L, I really must conclude that it just still can't autofocus as fast or in as low of a light, as the 70-200 f/4 (non IS)...even if the 70-300L is zoomed to f/4 at the wider end. The 70-300L has more trouble all around than the 70-200 f/4. I hate to sell it, but I can't afford to keep both lenses (also need to sell two of my Sigma lenses...I like them but I can't afford to collect 40 lenses like some of you guys).
The 70-200 f/4 non-IS, combined with the 6D, absolutely got the best out of the 6D's admittedly limited autofocus and servo tracking ability, in my experience. I've still not fully explored the 70-300L's servo ability, but given its single shot performance, I have to conclude it also cannot do what the 70-200 f/4 could do.
The 70-300L seems quite a lot sharper, except all the way at 300mm. It seems to have slightly more CA, but it's not a problem. It seems to have quite a lot more contrast (similar to the 135 f/2 and that's saying something, especially for a zoom!!)...And again, it most definitely is the most flare resistant lens I've ever used. The reviews that talk about this are almost understating it. I can't believe I'm pointing this thing nearly into the sun and no flare, no significant hazing (ghosting?).
The IS of the 70-300L can be spectacular (.8 seconds hand-held at 300mm)...but this is NOT the norm. And if I focus-recompose and I'm not in panning mode, the IS gets upset and I have to let it settle for probably 1.5 seconds before I snap the picture. It does seem to have some mirror slap compensation (which is impressive in itself at this price level imho)...But again, it's nothing like the best IS I've personally experienced so far, the 200 f/2L. That was in another realm...however the 200L's IS was probably only reliably repeatable while hand-held at 1/10 or 1/13 of a second, and faster. The 70-300L is similar at 1/30 or 1/40 of a second...at least at 300mm. In the middle, around 100-150mm, it might be repeatable down to 1/20 second. At the wide end it seems slightly better than the long end, but not as good as from 100-150mm.