What happens depends on how much sodium sulfite you add to the working solution.(posted 8583 days ago)Assuming you're using Rodinal 1:50, that usually gives up to a full stop less speed than the ISO rating when the film's developed to a "normal" CI (not the .65 Agfa's recommendations give).
Adding 12-25g/L sodium sulfite can being the speed back up a bit, by 1/3 to 2/3 stop.
Increasing that to 50g/L can give a slight decrease in graininess, but it's very slight.
I've never observed any difference in curve shape with HP5+ using 12g/L to 50g/L sodium sulfite. The curve shape is pretty much straight, essentially the same as HP5+ in D-76 1:1, Xtol 1:1 etc. Delta 100 and Delta 400 have behaved the same way although I don't recommend Rodinal for them.
The only film I've seen change curve shape with the addition of sodium sulfite to Rodinal is T-Max 100.
Its curve shape in D-76 1:1 and 1:3 is pretty much dead straight, while in Rodinal 1:100 a mild shoulder begins at about Zone X; with the addition of 25g/L sodium sulfite that shoulder goes away and the curve straightens right up.