I think they ARE identical, in technical sight!
The calipers are built in mirror symmetry, there is a “left” and a “right” one.
I really cannot understand why that has effect to the function. 
But there IS a difference.
Tried today two caliper pairs - only the rear caliper levers touched the caliper bodies. 
To the pads:
Assuming that the identification Hayes MX2/MX3 is correct - there should exist different types of pads : semi-metallic, sinter and kevlar.
http://torontocycles.com/Selling/Pads.html
I think it might be worth a try to test these types.
(And to figure out which type is originally used.)