Lower margin requirements mean they will be able to take higher positions. Say it isn't so.
It isn't so.
Let's assume the book for GC which JPM is running is ~1billion, and according to you they're net short. Let's assume they're net short for 100 million and dying for a reduction in margin so they can use that collateral to add to shorts.
Margin reduction of 13% means at most they can reduce their posted collateral by 130 million.
i.e. who cares.