not sure what your problem is pretty sure shadows cast time direct damage got buffed significantly mind blast is hitting like a truck and a half.
Just from the minor experience ive had with shadow on my toon if mobs are going to die fast or cant get full rotations in ill VT everything i see maybe MAYBE SW:P if enough time and fill with mind blast and mind flay or ms/mb spam. Simple problem you dont have enough time for your dots to do any real damage(so dont waste your time. use your highest damage abilities, hell you can even spam 3 mind spike then mind blast and still pull GOOD dps even tho your mana gets eat quick) your no different than a affliction lock other than they have a non cd direct damage filler.
You have options and the tools to not suck use them.