I agree with Mike. You should check out your neck relief. I have a hard tail Ibby that i used for several different tunings. I have to adjust the neck when going down to D or lower...Before changing the nut, you might want to increase your relief until the buzz goes away, then try the playability - I'd be surprised if you really need a new nut. The seems more like a relief issue, since the lower-tunes strings need more room to vibrate.
Maybe those telephone-pole gauge strings you were using wore out the bottom of the nut a bit and now lighter strings are buzzing??
I wouldnt jump to a new nut yet tho try putting a bit of graphite in your nut slots