Hi Bill,
I have not heard anything.
310.15(B)(6) allows 400 KCMIL THWN copper for a 400 amp dwelling service.
You could go traditional and use 500 KCMIL THWN, both fit in 2 1/2 per Annex C - Table C1 EMT and C4 IMC. In table C8 for RMC, 500 KCMIL would have to be in 3 inch.