Comments: The "sweet spot" is EI 1600-2000, showing the best combination of "real" speed, grain and acutance without excessive contrast.
As for really high speeds, one thing I used to do for TMZ in Microphen was add about 50g/L sodium sulfite. That reduced the graininess a bit but didn't make mush. I've tried it
with Delta 3200 in DD-X only so far, and it did make a difference. It's no magic bullet, but it may help in the quest for really high speeds and reasonable quality.
Also, since Delta 3200 is available in 120 there's a tremendous advantage to shooting it in medium format, especially 6x7. I bought a used Graflex XL w/f2.8 Planar specifically
for that; there's actually a big difference in quality between printing 6x7 and printing a cropped rectangle out of 6x6.