I use 2+2 to 100 for tray processing 10x8 films simply because it requires a shorter development time sat in the dark, 9 minutes instead of 15 minutes. There's a slight increase in grain but not temotely noticeable with 10x8
When I first started using Pyrocat HD about 20 years ago I found I got excellent results with 35mm HP5 in1+1+100, at a time of 15 minutes for film exposed at 200 in sunny conditions. This time seemed a bit long, so I tried the 2+2+100 solution. This brought the time down to 10 minutes. But it also resulted in a noticable increase in grain. I quite liked a bit of grain at the time and was also developing 35mm Hp5 in Rodinal. The grain from Pyrocat 2+2+100 was about the same as you get from Rodinal. This was definitely noticable in a normal sized print from a 35mm negative, but it didn't show up in a normal sized print from a 5x4 negative. So I standardised on the 2+2 +100 for my 5x4 negatives which I was developing at the time in the dark in an open tray. So shorter development times were quite handy. And Istopped using Rodinal because I found Pyrocat to be a much better developer.
Eventually I stopped using Pyrocat when I could no longer get the raw chemicals easily. and switched to ID11 and Perceptol at 1+3. Ironically, this put me back to long development times. But I really like the results...
Rate of development due to concentration has an effect on grain size one way around this used in a few developersin the late 1930's into the 1950's was to use Sugar or a Glycol to slow the rate of deveolpent. Ilford's Exttra Fine Grain Developer ID-44 used Dextrose (Glucose) and Salicylic Acid (Aspirin).