As of now, the best glyphs for Destro are Conflag, Immo, and Imp, and will remain to be in 4.2. However, given that pet classes (aka warlocks) will higher a higher chance to proc the effect from the legendary staff, we can assume that pet attacks will not proc the effect. In theory then, it may be a better dps increase to drop GoImp for GoIncinerate once a destro lock has the legendary. I say this because incinerate is a very large portion of our damage, and I would think that having the extra damage on something that can proc the staff's effect would be more beneficial than increasing damage on something that will go unaffected by it.
This is, of course, all theoretical until proc % numbers can be tested for destro. Again, it's only a possibility, and only once using the legendary staff, but I think it would be worth looking into once we are able to test the numbers.