Honestly, no reason to change it. No expert on hydrodynamic here, but both have about a 5HP/ton power/weight ratio (assuming 9500t and 47000HP for the Akula), so their speeds should be broadly comparable. Two knots more or less in one direction is just un-necessary.
If you want to improve the Akula, give us sensitivity or washout speed improvement. The game interface does a good enough job of modeling Russian sonar inferiority that you don't need a Nrd differential anyway... if it really is a SSAZ on the real Russian subs, whoever chose to keep it that way should be
shot
Or change the SS-N-27's airdropped torp back up to 55.
Or reduce the 688I's diving depth to 300m, since some sources suggest its dive depth is reduced to cram in the speed and reactor. With the Advanced Torp Mod, it has the effect of not allowing it to use depth to slow the approaching torp - in torp evasion, relative speed is important.