Consider not picking Sleep.
The thing is with everything that bypasses the HP system, you either allow it and they are often better...or you don't allow it. There isn't a good in between.
If a Fireball does 10d6 points of damage but the enemy gets a save for half and a Sleep spell just puts someone to sleep if they fail a save...the sleep spell could be doing 200 points of damage(effectively). If you limit the Sleep spell so that it only affects enemies who are at 40 HP or less...well, it's more powerful than a Fireball against any enemy who has 40 HP or less. If you also add a saving throw to it even against targets with 40 HP or less...it becomes worse than just fireballing them every time and is only useful for those rare circumstances where you want someone to go to sleep instead of die(and assuming you aren't playing with mechanics like 4e where you can knock them unconscious with a fireball). Plus, you have to lower them to 40 hitpoints to begin with, so you have to nearly kill them just to put them to sleep.
This is the same problem with all things that bypass hitpoints. Including all Save or Dies. If something has lots of hitpoints, it is better to use these abilities by far. In the vast majority of cases the idea is to take out the enemy(whether they are dead, unconscious, unable to move, or whatever, as long as they can't fight back and at your mercy to kill them off). That means if you allow Save or Dies to just take multiple rolls to pull off, you are not helping the rest of your party.
As an example, say it takes 3 failed saves to die from Save or Die. You hit someone with a "You are going to die attack". The next round, the enemy fails its save, but the rest of your party does 40 damage total. Then the round after that, he fails his save but the party does another 40 damage total. The third round the party does another 40 damage and kills it right before it makes it's last save.
Your save or die has done effectively nothing since it doesn't contribute to helping what the party is doing...reducing hitpoints.