With all due respect; you are wrong about much of this.
The variations with these films (proven in various tests in the past, when film still had some printing space in photo-magazines), is significant in many areas, but especially concerning:
- True speed
- Grain (yes!)
- Resolution
- Tonal response and sensitivity
- Reciprocity properties
- Curve response to pushing and various developing schemes.
The delta films from Ilford for example, have a much greater red sensitivity than Tri-X, Neopan or HP5+, meaning things like red lips will be almost white and pale, depending on the light.
- I find this extremely unflattering and after trying out the deltas, I simply gave it up for people. Even if the skin is smoother with higher red sensitivity, white and pale lips look SILLY!
HP5+ is a much nicer film and much more flattering tonal response, it has "old school" cubical grain structure and lower resolution than the Delta 400 (which is a newer film with T-type grain structure).
The grain means squat when you print a 4x6 inch print, but it does play a role in bigger prints.
True speed can be compensated for somewhat, but it limits pushing ability, a film with a true speed of 400 will handle a +1 stop with ease, Foma 400 will not, as it's true speed is more like ISO 250.
The look and feel and grain can only be emulated to a certain point, it will never be the same, because these films simply are different.
The only Ilford films I personally like, is HP5+ and FP4 (and I actually do buy those).