Further info about my process:
I agitate using Fuji's method: Once per minute.
I'm using a stainless steel single-reel tank with 200-210 ml of developer (barely covers the film).
XTOL was used stock (not diluted).
The T-Max 400 is batch number 0167 bought a few weeks ago and kept frozen, and the XTOL was mixed last week. So everything is fresh.
Regarding more dilution: The concentrates shown in those curves are similar to XTOL diluted 1+1, so I doubt dilution makes a difference.
Regarding agitation: The concentrates take about twice as long to develop, so they'll get twice as many agitations as XTOL. Since they develop at half the rate of XTOL, the behavior of the concentrates should be similar to agitating XTOL twice per minute. Yet the curves are identical. I'm curious to see what curves Michael R gets...