I'd want it to be balanced though - adding levels just means that enemies never hit you, and you've got powers that may unbalance the adventure e.g. paragon flight, and so on.
There would have to be the same sense of danger that you get with a decent, balanced encounter.
I don't believe there is any serious problem in 4e with unbalancing powers. At +9 levels monsters may struggle to hit certain high defenses, eg a Knight's AC or a Rogue's Reflex defense. They can probably still hit the lower defenses ok, by which I mean on a ca 15-17, as opposed to needing a 20. It's really only when you get into '20 to hit' territory that encounters become trivial for a solo PC; 5 monsters attacking with 19-20 to hit, 17-20 with a flank/CA, can still threaten the lone PC. I accept that 9 levels is pushing it, though.
Edit: For small PC groups, I find that in practice the following conversion works about right.
4 PCs: Treat as 1 level lower.
3 PCs: Treat as 2 levels lower.
2 PCs: Treat as 3-4 levels lower.
1 PC: Treat as 6-7 levels lower.
Eg: I have 3 5th level PCs. I can run an adventure written for 5 3rd level PCs.
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