Be aware that you don't have Epic Spellcasting, so if your party does you're not very useful. If Epic Spellcasting isn't being used, then it should be much better (I personally recommend pretending ES doesn't exist).
You definitely need Darkstalker... without it, everything at high levels detects you automatically.
Since you're using Knowledge Devotion already, I'd imagine archery is your best non spellcasting damage route. You might want to pump Iajuitsu Focus as well, in which case you'd want to be a Whisper Gnome for the stealth bonuses, free proficiency in the Gnomish Quickrazor, and access to the Trivial Knowledge feat (from Races of Stone). You might also want to look over the Factotum handbook and Guide to Proper Scouting handbook for more ideas.
JaronK