Hi! Welcome to Demo!
If you are a raider/progressing at all I would stick with Destro until you are well above the 520 iLvL mark and have UVLS otherwise it just isn't competitive at that level, but if you are playing Demo purely for the fun of it/want to learn more about it I can help a bit.
I am not exactly sure if its due to being at 8100 haste or if its needed before fitting 2 SB's before your 2nd hand of Guldan, but do some testing as later on this is the best filler between your opener you listed there. Your opener does look fine by the way but you should also be opening with Dark Soul + Imp swarm glyphs and any Profession CD you have. Here is a handy opener I use personally
/castsequence reset=target Hand of Gul'dan, Corruption, ***Insert Racial if you are Orc or Troll***, ***Lifeblood if herbalism***, Imp Swarm, Dark Soul: Knowledge, ***Insert glove name if you are Engineer***, Hand of Gul'dan, Metamorphosis, Doom, Doom,
/petattack
Please note the above macro and make changes depending on your race/professions. Also the double Doom is great IF you have UVLS and Glyph of Everlasting Affliction (you get a 2:20ish Doom with 100% crit chance) ***Note Glyph of Everlasting Affliction will be removed in 5.4***
When you complete this rotation you should have enough Fury gain from imps/spells to last most if not all your Dark Soul duration in Meta form, I will generally run out of Demonic fury after this and it is up to you during caster form to decide whether you are:
A) Casting Soul fire on Molten Core procs to build more Demonic Fury while weaving HoG's between Meta for max shadowflame dps
(Costs a lot of mana and you won't have the stacks for meta)
B) Casting shadowbolt while weaving HoG's between Meta for max shadowflame dps (Meaning you will have a lot of Molten Core procs for Meta)
That is the basic gist of Demonology.
If you have adds up that do have a substantial amount of health and aren't going to die soon cast Doom on them/when UVLS procs if you end up getting it.
I could go into more detail, but just ask for any more information!