Ok, so lets look at this again than as you are right that was pre GoLT, so In order for that gear to be better you would need to preserve 2 pc and from the aditional pieces gain 177 spellpower in not only spellpower but stats.
177=300*.39 (demonic aegis) +(200*.2)
Lets assume we are just replacing chest, pants and helm and 19 spelldmg gems only (meta is not a factor) and all spirit is factored in as spelldmg assuming .39 conversion rate (rounded down) all other stats will be added at the conversion rate listed below.
The question is how do you weight stats? Leaving them out of the equation completely doesn't work in the real world, but just comparing straight spelldmg we get this
Chest options are either:
[Gown of the Spell-Weaver] 129
[The Sanctum's Flowing Vestments] 156
VS
[Valorous Plagueheart Robe] 162
Helm
[Hood of Rationality] 170 spelldmg.
VS
[Valorous Plagueheart Circlet] 137 spelldmg
Legs
[Leggings of Mortal Arrogance] 183 spelldmg
[Leggings of the Wanton Spellcaster] 132 spelldmg
Vs
[Valorous Plagueheart Leggings] 137 spelldmg
Just using spelldmg you see a 73 seplldmg increase in the other BiS gear. This is not factoring in (as stated above) other stats of any sort which could sway it one way or the other (leggings of the wanton spellcaster for instance are, in my opinion, much better than mortal arrogance, if you can remove the hit from some where elsem also factor in set bonuses etc, you definitly gain much more than that on your gear set from the BiS. Zak if you have a better way of seeing that please let me know.