I have found Delta 400 very forgiving of overdevelopment. The first time I tried it, I used a time close to what 400 Delta (older version) required. Delta 400 requires considerably less development, so the negatives were quite dense, and I thought they'd be murder to print--not to mention, grainy. In actual fact, however, printing them was not difficult at all. I did have to use a lower contrast paper than I normally would, but I was very pleasantly surprised at how nice the prints looked. Grain clumping was not nearly as bad as I expected, either.(posted 8282 days ago)As for pushing it from 400 to 800 in Xtol, I'd try a moderate increase from your normal processing time, say, +25%. I'm assuming you are already using Xtol for EI 400.
My first thought, though, was why not use the same time you normally do and just print on a grade higher paper? Delta 400 has exceptional latitude, so I think you'd probably get a better result (sharper and finer grained) by pushing development very little, or not at all, then printing on a harder paper.