In terms of concentrated developers, do you have any experience of Rodinal Special / Studional / Hydrofen ? - this developer seems get attention and be well regarded, while also being recommended for faster films. I've had good results with Tetenal Ultrafin in the past for slow-medium films, but wasn't completely satisfied with it for HP5 / Delta 400 etc.
Studional seems to have been (conceptually at least), Agfa's attempt at an HC-110 type of developer, but semi-aqueous and using Triethanolamine rather than a Diethanolamine adduct. Like Ilford, there will have been a lot of commercial pressure on Agfa to release an HC-110 equivalent - just as Refinal was Agfa's equivalent to Microphen.
What does seem to have been the case was that by the time Ilford was working on DD/DD-X, it was becoming increasingly clear that alkanolamine based, highly concentrated developers were simply not capable of producing the same fine-grain/ good sharpness balance as a traditional sulphite containing developer (without a lot of potentially costly chemical processes etc to resolve the alkanolamine problem) - and it's pretty obvious that Kodak travelled along near parallel lines to get to Xtol. Ilfosol 3 seems to have evolved off in a slightly different direction, but the carbonate buffer on a PQ developer will further maximise the sharpness inherent to an optimal PQ ratio, whereas a borate buffer will generally deliver finer grain. Either way, Ilfosol seems definitely finer grained than most of the HC/HC-110 etc processed films I've had through my hands - but I wouldn't necessarily recommend it if you routinely want to push your film till the steam comes out. The other couple of things I would say about Ilfosol are: grain can be a bit more visible under some circumstances - not because it is coarser, but because it is noticeably sharper (likely from stronger inhibition effects - same thing that will limit the ability to push film in it, but give easier to print highlights); and you need to be a bit more wary about temperature conversions - from recall, one of the strengths of borate buffered developers is that they have really good development time linearity across the range of temperatures most people use.