The biggest difference I see between new Delta 400 and HP5+ is in tonal rendition (in "standard" developers).(posted 8264 days ago)Compared to HP5+'s dead-straight curve shape through at least 14 stops, Delta 400 has a distinct shoulder, modified slightly by the developer used. The result is that a straight print from Delta 400 shows quite a bit more highlight detail when both films are developed to comparable midrange contrast but highlight contrast isn't so low that it's muddy. The tonal rendition reminds me of _old_ Tri-X and is a distinct move away from the "modern" straight-line rendition that began with TMX.
I don't see any big differences in graininess, RP or acutance although there are at least slight improvements in all those areas.
The overall effect, though, imho offers quite an improvement over HP5+, so much so that I feel it'll be worth the effort to "learn" a new film and move to it for 35mm and 120.
I too would like to see it in large formats.