From what I've read the 9" rear end was homologated originally for the RS200 to cope with 650bhp+ and then adapted for use in the Gp A Sierras.
They were originally plate type ZF ones at first then viscous coupling on the Mk2 design.
Think you can get 4:27, 4.09, 3.82, 3.64, 3.42, 3.25 and 3.08 ratios. Andy G uses a VC in his Focus - the advantanges of the bigger VC is that it resists the build up of heat for longer without a cooler (although you could plumb one in ) and the diff can be rated much 'tighter' than a 7.5" with nickel plates - see the big thread on rear diffs for more info from the people who know like ian sibbert, markk, Tony Ryan etc.
The diff internals are much stronger obviously on the 9" as well.
As said though, they're not cheap.
Have you thought about the Titan plated diff ( supposed to be quieter than a Gripper ) that Mike R can get hold of in a 7.5" fitment? I don't know how much power/torque you run and how you drive though