First: why are you using Khem as darkness? Why not a geared dps?
Second: if you're a sub, you have free re-specs...of course you should try madness. Do a little reading, fart around with a spec, and go kill things! Madness or madness/hybrid are both a lot of fun and with a dps comp ( I use HK) mobs go down very quick. If you don't like it, spec back.
Third: are you going to tank end game content? Make sure you run Battle of Ilum and False Emperor to get the practice in
How is BoI hard to tank? You skip 9/10th of the fp, and there is one ability to interrupt on the boss thats it.
Dual dps will always be the fastest leveling content.
On the other end, sintank can solo some H4.
If you persist with Darkness build, next 10 points are going into madness to go up to chain shock.