Definitely I wouldn't augment the number of picks of SPPI.
Consider that if you do this, you can have TWO party members with extremely low casting time: a mage with the robe, and a sorc with too many picks.
If we go for the solution proposed by the bigg and myself, only one party member at a time will be able to have extremely low casting times, so the equilibrium will be preserved.
The more I think about it, the more I'm convinced
Lets see the details then. We have two items and one HLA with direct spellcasting speed decreasing effects: The Robe of Vecna, The Amulet of Power and Spellcasting Speed Increase.
Here are the possible combinations of the three, assuming one plays with a mage and a sorcerer in the party (using your suggested values):
1.: the mage has the robe and the amulet (-5), the sorcerer has SSI (-3).
2.: the mage has the robe (-4), the sorcerer has the amulet and SSI (-4).
3.: the mage has the amulet (-1), the sorcerer has the robe and SSI (-5).
4.: the mage has nothing (0), the sorcerer has all the boosts (-6).
And here are the same combinations using my version:
1.: the mage has the robe and the amulet (-5), the sorcerer has SSI (-5).
2.: the mage has the robe (-4), the sorcerer has the amulet and SSI (-6).
3.: the mage has the amulet (-1), the sorcerer has the robe and SSI (-5).
4.: the mage has nothing (0), the sorcerer has all the boosts (-6).
As you can see, there are really small differences, especially when it comes to extreme values. The lowest casting time never surpasses -6 (in the case of the sorc), the total amount of bonuses is 28-32.
Also, in my suggested version the sorcerer would have to sacrifice 2 extra HLA points to get to this level, while in your version they'll get to their "maximum" 2 levels faster.
Another point is the nature of the robe itself. Until now, the Robe of Vecna was loved because of it's powerful boost to spellcasting speed - namely ONE effect, the others were there to make the deal sweeter. If we change this artifact by replacing it's one big bonus to a score of minor extras, it will lose it's potential, I'm sure - inferior
AC, save,
MR, spell bonuses won't compensate for the loss of the massive spellcasting speed increase. That is why I suggested adding an equally massive bonus to sorcerers as a replacement of the entire effect instead of simply reducing it to a seemingly insignificant value. That way we could keep the two classes balanced, we would keep the robe's appeal to sorcerers as well, and would make sorcerers harder to get to their full capacity at the same time.
Sounds a perfect solution to me...