What difference can we expect between Pan F and FP4+ ?
A tiny bit until you make large enlargements, enough to be discernible, but Delta 100 would fall in between the two, and that would be a more valid comparison along with Tmax 100.
There have been no proper comparison of Pan F sheet film, to other films, at all. All the testers have used their film with no direct side by side comparison, so that isn't helpful. There's a need for proper qualitative testing, now the sheet film has been released that might happen.
It is also odd that Ilford themselves have not made any effort to market the benefits of Pan F as a sheet film, It seems they now rely on YouTube content creator, and influencers, for their marketing.
When Kodak test new films they use world respected photographers, like John Sexton for instance for Tmax films, and he was critical about Tmax 100 only having an EI of 50. Kodak had the ASA test revised so Tmax 100 just passed then rushed out Tmax 100.
Now that is not to knock Pan F as a sheet film, or 35mm & 120, I have a large stock of 5x4 film, over 850 sheets, so I won't rush to buy and 5x4 or 10x8 Pan F, I still have quite a bit of EFKE 25.
Personally I always test films I've not really used before in smaller formats first, before using them for LF, and I'm predominantly an LF shooter. I have 36mm & 120 Pan F (which I was given), and it could be the ideal B&W film for my 6x17 camera. That's where i might see a serious improvement in terms of sharpness & fine grain, and fine tune for tonality.
What you have to remember is how good 5x4 HP5 is, enlarged to about 20x16, and Delta 100 to a bit larger, it's similar in a loose way to a Post card size print off a 35mm negative, in terms of the degree of enlargement.
Why would I shoot a 50 ISO film? Well I do with EFKE 25, 25 is the Tungsten light ISO, Daylight speed is ISO 50, oh and in the past Tmax 100 @ 50 EI. it's because I like to shoot f22 and slow or longer shutter speeds, that captures movements in grasses trees, clouds, etc.
Bottom line test differences in films with smaller formats
Ian