The power curve has nothing to do with it. The effect is an all or nothing effect, adjustable only in terms of save DC.
Power curve has the following effects on save-or-die.
1) The DC has to be set correctly to match the expected character level and saving throw. While this does encompass power curve issues, the way its done does not. Begin by deciding what chance of success you want the spell to have against a high fortitude character, reference your game designer chart of expected saving throws at any given level, and create a new chart of benchmarks for save-or-die.
2) The availability of countermeasures changes by level. If the attack spell "BANG! You're Dead!" is at 5th level spell, you decide whether the ward spell "No, You Missed!" is a 4th or 6th. The problem is, the more save or die spells you have, the less likely the players will ever have the particular ward spell ready at the right time.
3) The ability to come back from the dead is more common at high levels. Raise Dead functions as a repair spell for spells that kill you.
That's... pretty much it. Those are the only level-relevant things that affect save-or-die. I know you can spin out scenarios in which other level-relevant things might matter (higher level characters will kill a given enemy faster and will be exposed to save-or-die from them for fewer rounds), but nothing else relevant to the save-or-die aspect of the monster itself.
Now, the DC thing doesn't really count for me as a power level issue. If the problem with save-or-die was that the saving throws were set too high, we could rebalance them in about 10 seconds. I don't think it helps though. I think I'd find a monster with a 5% chance of killing me outright per round even more annoying than a monster who can do it with a 40% chance, since dying to the first one will feel even more petty than the second. And while there may be a perfect point which optimizes the playability of save-or-die effects, they'll still be save-or-die effects with all the things about them that bug me and my players.
The second scaling effect relevant to save-or-die is a particularly unsatisfying one. Using a Wall of Ice to protect against arrow fire is pretty cool. Using a spell which has no effect whatsoever except to make the enemy's spell not work is not cool. I used the spell names "BANG! You're Dead!" and "No, You Missed!" for a reason. If the save-or-die is the monster's only good attack, and the spell is available, they turn climactic encounters into cake walks. When the monster has other attacks, the spell becomes an annoying ritual who's only real purpose is to use up a PC's spellslots. When the spell ISN'T available, it might as well not exist. I find this to be an unsatisfying, pseudo deus ex machina way of handling matters. And in any case, while it is power curve relevant, in the existing game it isn't very much so. Death Ward is a 4th level spell. Protection from Evil, a similar style spell for dominate attacks, is a 1st level spell. In general, these spells come out ages earlier than the attacks they protect against.
Finally, we come to the 3rd issue, Raise Dead. I don't have anything to say here other than that I don't like turning death into a status effect. I think doing this ruins the one good thing that save-or-die has going for it- the ability to instill terror into the players. And of course there's all the usual wonkiness. You can't Raise someone who was killed by a Death Effect, but you could have used Death Ward, and you can Resurrect them, except they'll lose levels, on, and on, and on.