I had a good feeling it would be the 800mm; as stated in an earlier thread. But they better make it appealing. Not just compete on par with the Nikon version, which by the way is heavier than the current Canon 800mm.
I don't know why it would need to be priced over the Nikon version. Since it uses elements that are smaller in diameter than the 600mm IS II. Less fluorite element should mean cheaper to produce. Also less material for the barrel, etc. But, more often than not these decisions make little sense.
Cams: Canon 1Dx, 6D, EOS M
Zooms: 17-40mm f/4L, 24-70mm f/2.8L II, 70-200mm f/2.8L II; Primes: Sigma 35mm f/1.4, 40mm f/2.8 STM, 50mm f/1.2L, 85mm f/1.2L II, 100mm f/2.8L, 135mm f/2L, 300mm f/2.8L IS II, 600mm f/4L II
Support: Gitzo GT4542LS/GT2830/G2258, RRS BH-55, Wimberley WH-200