Better read the directions. That's *3* minutes in each bath, not two. I usually rate the ISO 400 Tri-X at EI 1600, but some do prefer 1250 or even 1000, claiming improved shadow detail. And it's still a good idea to use a water rinse (no acid stop, please, due to the carbonate based Bath B) between the developer and fixer, t9o prevent carry over from shortening fixer life (though the carry over is much less with Diafine than with most others, because the developer in the emulsion is almost entirely exhausted in the three minutes spent in Bath B).
Some find the negatives "flat" but I don't see a lot of difference (in scanning -- don't have my darkroom set up to print yet) between Diafine negatives and those processed in HC-110 to what I consider "normal" contrast.
Be prepared to see some magic -- Tri-X is the film for which Diafine was created. Everything else is just along for the ride...