As I recall in that interview last year, he said the 300+100 was his best workout for 400. He was also asked what the best workout was for MaxV and he said 300s.
There are some old forum discussions around on 300s with particular emphasis on Asafa. Charlie mentioned Asafa doing 300s in 37. I think Doc mentioned 3X300 in 38, same thing for Dix. John Smith has had people getting down to 35-36. PJ has mentioned Bolt doing 350s and 450s, but I haven’t seen mention of pace. This is all mostly intensive tempo of course, with accels (or hills or sleds) along with it (not on the same day of course).
I’ve had better luck with 3X2X150 (becomes 4X2X150 later in GPP) with 60 sec between the 150s, but run at special endurance pace.