2/3rd stop definitely makes a difference and especially in the UK.
It's difference between shooting at under 1/1000s or over 1/1000s
Plus the point is it'll be much sharper with TCs
He's done a comparison against 200-600mm, even with TC it's sharper
View: https://youtu.be/93Cof3kVhTA?si=K2L7q5T7pgp5hxfr
300mm f2.8 can't go to 800mm with 2x, plus it costs 50% more and it's not a zoom.
i also don't see it as a replacement for the original 100-400mm. It's more a good upgrade for people shooting with 200-600mm or 400-800mm, it'll be mostly sharper and lighter by a fair margin.
Horses for courses and all that