I have been driving around with my diffuser installed for about a week now.
I must agree with
@D-resonant that the main thing I feel is less drag.
The car just seems to coast much easer.
This, to me means that overall I should get better gas mailage (not that I was worried about this before)
Where I don't agree is any clearance issue between the diffuser and the twist beam.
I took a very close up photo after the install of this space and as you can see, with the Ebach bar there is plenty of room.
I do not see why the Hotchkis bar should not fit.
From what I have seen in photos the Hotchkis bar sets in the exact same place.
The only difference is that the Hotchkis bar bends up in the center but up should be an issue.