There is a chapter about the previous 0.25g phenidone version in the Kindle book I have by John Finch "The Art of Black and White Developing". He gives photographs illustrating that the grain is quite a bit finer than with Pyrocat HD. A rather enthusiastic review does not cover the downsides.
It would be interesting to hear later from Andrew how 510 Pyro compares on sharpness with Pyrocat HD, John Finch recons it is sharper, when I tried it I did not think so.
Therein, it seems to me, lies the problem of it ever being possible to state with a degree of getting overall consensus on almost any attribute of a film such as sharpness despite what photographs would seem to reveal
In his "Battle of the Giants" series of videos John Finch concluded that Pyrocat HD won. In fact the final contest excluded 510 as it had lost in a previous round. The tests involved a non-staining developer as well. However John subsequently withdrew one of the videos involving 510 Pyro and I believe he intends to re-do it.
It seems he may have decided that his print comparisons in the 510 Pyro contest were not quite a fair comparison
That is not to say he has changed his mind on his final conclusion about pyrocat HD being the best but I look forward to his re-run involving 510
pentaxuser