Kodak's times for "Large Tank" (which is what the 3 gallon tanks are) are about 15% longer than for "Small Tank" (like the Nikor tanks). Well, that's for Tri-X, which is a cousin to HP5+.
Agitation technique is much more critical with racks in large tanks. Easier to get non-uniform results.
The difference in processing times is due to different agitation routines Kodak specifies for Deep Tank processing.
I develop sheet film in large quantities in deep tanks all the time. I have found that agitation technique needs to vary depending on the film being developed. Some film requires very little agitation to produce beautiful negatives (the old Forte 200 films, for example) while others, such as the Fomapan 200 films require almost constant agitation to avoid streaks.
I say this with the constant that the developer remains the same.
I always use a 4 minute water pre-soak for my b/w sheet film processing in tanks. I have a separate tank that I fill with water, and put the film hangers in (in the dark) for 4 minutes with agitation before I go into the developer. This technique has solved 99% of uneven development and streaks. I would recommend this technique also for roll-film processing in deep tanks.