Found an HP5 curve. Not sure whose data it was (Someone like ic-racer with a Wejex).
It’s got long toe and sag in midtones. So Drew’s probably not wrong on that description.
Firstly, that's for a 1.1 CI, so of course on initial visual inspection, the curve shape looks a little off kilter. Secondly, if you properly normalise the data against other 400 speed general purpose B&W films (e.g. 400TX, TMY-II, Delta 400) you'll find that they all have very similar (to the point of near identical) toe shapes, which are regarded across the industry as 'short toe' for these purposes (if not in Drew-world). If your sensitometry is up to scratch, you should find that 400TX's toe will plot in line, if you allow for the shadow speed differences. They also have similar mid-tone characteristics, with HP5+ tending to sit in the middle between 400TX's slightly bumped up mids and TMY-II's slightly darker mids - if anything, HP5+ plots a straight line between them - this is also clearly obvious if you print them in the darkroom. Finally, D-76/ ID-11 seems to still be the reference developer that was used for these materials, so it should deliver the optimal results that the materials were designed to deliver. And finally, much as it will shock those who are desperate to clutch their pearls, Pyrocat HD is effectively a D-76 derivative too, so there are no excuses allowable in that direction either.
These films are all general purpose materials, and despite errant claims from Drew, Ilford did actually publish extensive reference data for the standard separation filters and the use thereof (and were quite big players in the print world, which was probably why they were merged with Anitech (Ansco) by International Paper). Extrapolation from what elements of a very small part of the industry did (aka late era dye transfer) need to be informed by the fact that an awful lot of that specific published advice is essentially a dressed up form of 'push the timer start, let Kodak do the rest'. At the end of the day, they chose what the main process supporter told them to use. There were many separation processes and essentially it all boiled down to general purpose films and general purpose film developers, be it Kodak, Ilford, Du Pont, Ansco, etc, etc. And finally, HP5+ is effectively supposed to be a 400TX competitor, thus the similar toe/ curve, with better shadow speed and highlight gradation - both of which trip people up in different ways.