However, I think they sort of made it so that certain mobs required them to be claimed for increase in drop rate or something. Maybe I misread the thing when they nerfed people farming tons of mobs using -aga spells.
I think thats the nerf for procing. Supose youre in the magic oroc window, casting sleepga wont be procing many of the due nerf, aside the targeted on, which will proc at normal rate.
Considering drop rates, generally cleaving always will produce better results than single target farming. Up till uncommon drop rate item category, TH8+ wont make up for 2 monsters. Above uncommon drop rate though, extra TH can potentially make up for 3+ mobs cleaved, so its a factor to consider if there are restrictions on mobs availabiility (for example, sphere items drops).
Though, i must say, in practice even cleaving those in general nets better results than stacking TH.