why just separate voidform from insanity completely? make voidbolt an insanity dump thats always available.
voidfrom would no longer replace shadowform. it would now read
Voidform 60 sec CD
Pulls power form The Void, increasing haste by 15% for 20 secs increasing by 1% every 4 secs.
Surrender to madness could just triple the duration and CD.
void bolt could cost whatever maths out to be 1 every 4-5 seconds or so without being to intrusive.
if that's the case dots would need a duration buff.