I did some testing myself about the librams.
They dont cause a GCD out of combat, however in combat they seem to trigger one. And if your spells are allrdy on GCD and you swap, that GCD seems to refresh, so if you use a spell, then half a sec later swap libram, the GCD will refresh making another 1.5sec GCD, having a 2sec GCD instead.
Dunno about macroing, it should swich instantly whith the cast, but id susspect there would be some delay every time (less than half a sec but still). if this would contribute or not to total tps done, i have no clue.
Its a good starter tho to open with holy shield (with its libram) swap to LoO then pull, judgement and shieldslam.
Then ofc go back to Holy shield libram.
About the shoulders. ill prolly keep my T7.5 over deconstructor atm, since im using 2part bonus of T7.5 untill freya gloves prolly.
7.5 offers more avoidance for a little sta and lots of hit (wich i personally think i allrdy got loads of, 6.5%)
Altho im gonna loose the hit when i get a few other parts, ill for sure swap to deconstructor shoulders+ freya gloves once thats an option.
Also, i dont think its gonna be worth using T8.5 for SBV gear.
The bonus gives 225bv with a 50% uptime.
With only 2parts giving BV, id rather take those parts, then the Unbreakable Chestguard
http://www.wowhead.com/?item=45334 aswell as Wyrmguard Legplates
http://www.wowhead.com/?item=45842 with
http://www.wowhead.com/?item=40581.
Just by using those instead of the avoidance directed T8s, you get 219 more passive BV, isntead of a 225 with 50% uptime (cba to take str into account, but its still not gonna make the 4stbonus better imo)
IMO, 4setbonus is just badly itemised. and with gloves and shoulders beeing so bad compared to other available options i see no reason to actually have it, atleast not in your wishlist gearsetup. Using 8.5 head/gloves if you cant reach freya hardmode would be understandable imo. But no "MT" should EVER be using T8.5 shoulders over T7.5.
Thinking about it, we got 3/5 Block oriented parts in T7, and we get 3/5 Avoid oriented parts in T8. No set should ofc be 100% here or 100% there, blizz wants us to discuss the options to sacrifice "better" parts for bonuses etc.