I have 2 warmoth vintage modern strat necks (a third on the way) that came with corian nut (1 11/16). Both were cut very well - no binding up or pinging when tuning/bending. (One guitar has a hard tail and the other a vintage trem) I find that the spacing is even, aligned properly and the depth (action) of the slots are pretty much where it should be - I use hybrid slinky EB 9 - 46. I feel that the corian is a good choice, so is the Tusq when slotted well.
You may need to file out a tad bit more on your problematic strings (might just need to widen the ends of the slots that are closest to the headstock and tuners). If you don't have proper nut files - I would take it to a guitar tech. It is really easy to mess a nut up with agressive filing or using improper files.
Pretty hard to determine what the exact cause is here but I don't think that its simply the material (corian) of the nut causing your problem. I can't imagine that a decent tech would charge you very much to address the issue. As a matter of fact, The last time I took my acoustic in for a setup and strings, they lightly filed the nut for me to fix this very same problem. This was included in the original setup/strings cost - no extra charge.
I would only seriously consider replacing the nut if you feel that it is not slotted correctly - that is, its NOT evenly spaced between each string - the slot depth is too deep (strings are very close to the fretbroad = buzzzzz) - Slots are not aligned properly ( E & e strings run too far or close to the edge of the fretboard). You would pick this up quickly when fretting chords in the home/open postion as it would just feel like you are misfiring with your fret hand when changing chord shapes)
Good Luck
Kurt
