Keep Shadowburn its an insta and comes in very handy in PvE and PvP, its imo the best Talent in any of the trees. Despite Nightfall giving insta Shadowbolts, i prefer Shadowburn for the simple reason you dont have to rely on luck to use it, and its low mana as all talent spells are.
Personally i tried Nightfall and didn't find it that useful, it didn't often proc in PvP, in PvE i usually had the situation under control and so never needed to use a Shadowbolt, i typically use Life Tap (turn health into mana), then Drain Life, which does damage with no loss of mana, once all 3 dots are on a mob i'd do that twice then finish with a Sword and Firestone. This is a very mana efficient way of killing mobs.
Grim Reach and Destructive Reach are both very good talents, I have Destructive but thats cos I am heavily spec'd into Searing Pain. Either are useful, both would rock.
If your taking Improved Imp then you really want to be taking Improved Firebolt, makes an Imp deadly, at my lvl he does roughly 100 damage a second, not including crits.
Fel Dom and Master Summoner are a nice combo, i'd very much like to go for them, however you have Improved Imp/VW/Succubus, i'd recommend only taking improvements in Imp and VW. Typically i dont use my Succu for xp as it cant hold aggro or for PvP cos of the Soul Shard cost. You'd prolly imo be best off investing those 3 points elsewhere, Ruin if you can get it is a very nice ability, especially when you use your DD spells in combination with the appropriate debuff i.e Curse of Elements or Curse of Shadow.
Seems to me that your set up nicely to do a spot of drain tanking, on the whole its a good xp set up from what I have heard, and mana efficient when use right. You should be able to go through mobs with a minimum of downtime, however imo some tweaking might make it better.