Zealot drops/warp ins are economically viable. I mean, first of all, you'll get worker kills, 100 minerals per worker (50 for lost worker, 50 to remake) and a zealot will more than easily pay for itself once it gets into the mineral line.
In addition, he'll have to move his army back (or at least some of it) to deal with the drop. This way, you also get scouting information as well as the added bonus of delaying any pushes (at least for a few seconds). This is when you're right about pushing and warping at the same time, the army is split up meaning that it will be easier to take out.
Also, I don't know if there is a strict definition for a 'death ball', but from my perspective I think 200/200 supply. You can't warp in at 200 supply.
As I see it, unless it gets shutdown completely without doing economic damage, I'm pretty sure that a zealot/dt warp in is economically viable. That being said, I'm not pro, I could be wrong.:P
Unless your zealot got charge, or else they can just easily run away their workers and dropping will become very cost inefficient. Warp prism costing 250 minerals doesnt help much. If you did watch the match between HoSeo Sage Vs siriusfOu, he uses proxy pylons + phoenix high ground vision to warp in chargelots to harass.
Overall i think warp prism strats are quite workable on PvZ, while its alot less effective on PvT.
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