Here is small test in the vain of real world tests. Its not that scientific but just comparing Xtol at different dillutions. Both are shot on short rolls of HP5 and processed my usual way. Photos ware taken hand held and exposed as 400 with OM4 and 50mm lens.
I scanned them flat and added the contrast and sharpening. I also cropped a bit as always with crappy negative holders. Usually this is my minimal post-processing that i do to all my photos and when demanded i add a bit of contrast or play with high lights and shadows.
Anyway the results are quite similar, the 1:2 dilution seems to give bit more from the emulsion speed and give a bit more grain. But the differences is quite minimal at least for my eyes.
I have few short rolls taken on this same occasion, i will process them same time soon and write something like this on blog with the tag TestingHP5