It will definatly be interesting. Destro will be top DPS is these go through due to how the spec buffs fire damage and has no execute phase and uses incin for the entire fight, unlike demo (incin only on MC procs during Decimation for it's execute) and Aff (nightfall procs and occasional hard cast to keep 5% crit debuff up if no-one else is providing).
The Destros uptime will be higher over the course of teh entire fight going by that, thus DPS should be higher, however we don't know the stats of the set bits and off set bits. It just may be that going 2p T12 and rest off set could be a DPS boost over pure 4p T12, especially if you want to do drain life aff.
Something no-one else seems to of considered.
EDIT:
It does suck however that soulfire and drain soul are not included int eh 4p. If they add those 2 to the list of 'procable' abilities it will be more rounded. However for all we know they could be planning on buffing or nerfing abilities in 4.2, so all we are talking about could be a waste of time in a few days if/when they release class changes for 4.2