Nah, I do fine with Destro. I get around 27-29k on most fights. (My ilevel is 665 right now.)
But as always that a good dps should do as you pointed out, You always want to see what you can do on your share of a given fight to pump out as much you can! 
I tried trying to get your 3 macro thingy(3 buttons.) from here to work:
https://wowlazymacros.com/forums/topic/gnomesequencer-for-demonology/
But I can seem to get Shadow Bolt to fire on the first macro.
/targetenemy [noharm][dead]
/petattack [@target,harm]
/petautocastoff [group] Threatening Presence
/petautocaston [nogroup] Threatening Presence
/use [noform: 0] Metamorphosis
/castsequence Hand of Gul’dan
/castsequence [nochanneling] reset=target/15 Corruption,Shadow Bolt,Shadow Bolt,Shadow Bolt,Life Tap,Corruption,Shadow Bolt,Shadow Bolt,Life Tap,Shadow Bolt,Shadow Bolt,Corruption,Shadow Bolt,Shadow Bolt,Shadow Bolt,Life Tap,Shadow Bolt,Corruption,Shadow Bolt,Shadow Bolt,Shadow Bolt,Shadow Bolt
Here is 2nd Macro: (Which seems to work fine I think.)
/targetenemy [noharm][dead]
/petattack [@target,harm]
/petautocastoff [group] Threatening Presence
/petautocaston [nogroup] Threatening Presence
/use [noform: 1] Metamorphosis
/castsequence reset=target/5 Doom,null
/castsequence [nochanneling] reset=target/5 Touch of Chaos,Demonbolt,Demonbolt,Demonbolt,Demonbolt
/castsequence Dark Soul: Knowledge
/castsequence [combat] Imp Swarm
/use [combat]13
/use [combat]14
And the 3rd macro: (Not sure if this one is working correctly.)
/targetenemy [noharm][dead]
/petattack [@target,harm]
/petautocastoff [group] Threatening Presence
/petautocaston [nogroup] Threatening Presence
/use [noform: 0] Metamorphosis
/castsequence Hand of Gul’dan
/castsequence [nochanneling] reset=target/15 Soul Fire,Soul Fire,Soul Fire,Life Tap,Corruption,Soul Fire,Life Tap,Soul Fire,Soul Fire,Corruption,Soul Fire,Soul Fire,Soul Fire,Life Tap,Soul Fire,Corruption,Soul Fire
If you wondering why not I just dont simply use the GS addon? Just scares the hell out of me of getting my account banned. Just taking a very huge risk to take with your main account. If they up and you know what I mean? Decide to ban people from using it.
But any help would be very appreciated…