RF 800 f/5.6 and RF 1200 f/8

[Kayaker72]

I think this is analogous to the RF lenses.....many well known photographers recommended the EF600mm f/4 plus teleconverters over the EF800mm f/5.6

Exactly. The 600 mm (and I'll include 500 mm as well) seems like a sweet spot in the lens design. This may be a combination of technical and economics, but on both EF and now it looks like RF, a 600 mm plus 1.4x TC appears to be a great alternative.

I often find myself shooting at F/4 over having a 1.4x just to get that extra stop of light.
But the big question, will any gain be worth $4,000.

I start at a point where I assume Canon are not dummies. They would not make a sizable investment in 2 new lenses if they did not think they could differentiate them. Granted, I've seen some posts where people are opening wondering how much overlap there is between the RF 400/600 and these two new lenses (maybe not much as certainly the exterior of the 400/800 and 600/1200 is similar).

But, getting back to the basic question, what would the differentiator be? Why would Canon think someone would pay that extra $4k? If it isn't IQ at a given focal length, I am wondering about AF speed. Certainly, with a 2x TC, AF speed takes a hit. I've never noticed much of a hit with a 1.4, but I am wondering if the new lenses have blazing fast AF.

Regardless, these are high priced specialty tools. Not something I am looking for. But give me the rumored 200-500 f/4, and a built in 1.4xTC and I might pre-order it.