I always presoak when I use the Jobo, which is for everything except Diafine which I do use, but not for sheet film (yet.) No problems with the pre-soak. I use the five minute just as Jobo recommends.
I have read people saying that the dire warnings against pre-soaking with Diafine are likely not true and it would still work fine. I don't know. It works fine without the pre-soak so I'm not sure why I'd try that, unless maybe I was doing sheet film in trays. Doing sheet film in trays, manipulating the sheets without scratching in total darkness, always made me nervous to think about so I never even tried it. Before I got the Jobo I did my 4x5 in deep tanks with hangers. This worked fine, but takes a lot of solution and makes temperature control difficult. I have considered trying 4x5 in Diafine using those same tanks and hangers, which I still have.
To address someone else's point, the speed increase with Diafine depends greatly on the film. Modern films get less than older ones, sometimes none. It's been decades since I've used FP4+ in Diafine (not sure it had the "plus" in those days!) but at the time I seem to get an effective speed that the Diafine box said, but I think that was faster than the current box says. I would say that if you shoot Tri-X (TXT, I've not tried TXP) at box speed and develop in Diafine you probably will not like the results, whatever a densitometer says. The box says 1600 and in daylight that looks good. In tungsten light, which is likely when you need that kind of speed, that's underexposed with modern Tri-X. It looked pretty good in the 80s. Modern Tri-X I shoot at 1000 under tungsten light in Diafine. This is my favorite combination for situations when I need more than 400 but don't need TMZ or Delta 3200 and one of two combos where I use Diafine. The other is with Pan F+. It picks up a bit of speed from box, or at least lets you shoot at box speed when the film might like slower, and the compensating effect of two bath development tames the sometimes high contrast of the film nicely. So, oddly enough, I use Diafine for my 35mm and 120 films when I want either a very fast, but not the fastest, film or when I'm using the slowest one that I ever shoot. Probably 75% of my black and white is Tri-X or FP4+ shot at box speed and developed in D76 1+1 though.
All of which has absolutely nothing to do with the question in the original post.