OK ready for some maths?
Say DPK is 1000 on an item:
Current system (hypergeometric probability)
Killing 1000 = guarantee
Killing 500 = 50% CUMULATIVE drop rate
New system (binomial probability) (DPK reduced to 500):
Killing 500 = 63% chance. You can work this out as 1-((1-(1/500))^500)
Killing 1000 = 86.5% chance to get the drop. You can work this out as 1-((1-(1/500))^1000)
At 500 kills, you will have more chance. At 1000 kills you will have less chance as it will never be 100% chance for you to get the drop.
The point at which these two coincide is 800DPK on the old one or 400DPK on the new one, with both being 80% CUMULATIVE chance.
TL;DR You kill 80% of the DPK, you are better off on the old system. If you get it before 80% you are better off on the new system. Most of the time you will be better off on the new system provided they cut DPK by half.