Curves shmurves. 400TMax has the longest, straightest response curve of any film I've ever used. Take a look at this photograph, shot on 8x10 400TMax sheet film. There are approximately 12 zones between the shadows in the vaults and the highlights in the rosette. The proof of the pudding is in the tasting.

Having said that, I should also point out that I've heard that there's a difference between the TMax roll film and the sheet film; namely, that there's a dye in the roll film that blocks UV. I don't know if that is true or not. Maybe Ron knows. If true, it might account for your results differing so drastically from my experience.