2+2+100 will give you more grain which you won't notice in 5x4 , but you would if you were using 35mm film. 35mm HP5 would come out with as much grain as if you were using Rodinal. Dev. times will be shorter.
The stronger dilutions are typically used for alternative process negs to achieve the higher contrast needed within some normal development time. For example, I use 2:2:100 when developing for pt/pd prints.
Also, since A is essentially the developing agent and B is the accelerator, some folks play around the the standard dilutions to achieve some specific goal. Personally, in all the years I've used Pyrocat-HD I've never used anything other than 1:1:100, 2:2:100, or, more recently, the diluted mix used with Steve Sherman's EMA process. It's actually pretty versatile stuff; Sandy King once told me you could probably go to 1:1:400-500 and still get good negs.
I only use 2 + 2 to 100 when tray processing 10x8 sheet film.
One of the advantages of Pyrocat HD is it's ability to handle all sorts of contrast, in my case extremes such as shooting in the foggy Cornish sea mist or the harsh glaring sunlight of Turkey & Greece and then everything in between. More importantly the negatives are always easy to print and scan just as well.