Well, I have the 2x II and the new 2x III along with the 300 2.8L IS lens...but on a 1d4 body. I only recently got the 2x III and have only done some preliminary tests with this combination. I see an improvement in focus speed and contrast, and IQ looks slightly better with the new 2x III, but I haven't really done any scientific tests. It's good enough for me to keep it, and I'll probably sell the 2x II at some point.
Cameras: 1DX, 1D4, 20D, 10D, S90, G2
Lenses: Canon 10-22mm, 16-35mm f2.8L II, 24-70mm f2.8L, 70-200mm f2.8L IS, 300mm f2.8L IS, 200mm f2L IS, 50mm f1.4, 50mm f1.2L, 85mm f1.2L, 1.4x TC, 2x TC, 500D macro, Zeiss 21mm
Lighting: 580EX, Elinchrom 600 RX's, D-Lite 4's, ABR800, 74" Eli Octa, 100cm/70cm DOs, Photoflex Medium Octa and reflectors, PW's, Lastolite Hilite, Newton Di400CR bracket