Tom I do not really know how to answer your first two questions but I replaced original Dunlop Series 1 4.2 calipers on my 68 with the solid rotor 4 piston Wilwood kit from Classic Jaguar in Austin, see link below. The one I got was to use with the original rotors. I chose it since was an easy all bolt-on conversion that appeared to have more pad area than the originals, could easily go back to original and at the time was about the same cost as having my 4 original Dunlop pistons sleeved and re-worked. At the same time I did have the 4 Dunlops re-worked on the rear. There was a good improvement over my originals but my originals were in pretty tired shape when I bought the car. I think it brakes well and the balance is fine for me but I am not the most aggressive driver. I have not driven an E-Type besides mine so I probably am not the best to say how the Wilwoods are vs well sorted originals.
My bet is the guys at Classic Jaguar could answer your questions. My memory is they were very helpful 10+ years ago when I was inquiring about and ended up buying their kit.
http://classicjaguar.com/cjparts/eng_brake.php
David Barnes
68 E-Type FHC