what can a mk2 escort diff run ??? power??

I know someone running a 260bhp rover V8 through one quite happily.

I'd be more worried about the halfshafts trying to twist themselves to death, rather than the diff
if your planning on running serious power
Alot of the big power Escort lads run uprated halfshafts

I ran 260bhp through one for 3 years with no problems but that was turbo bike engine so not massive torque.

Also depends on how much traction you have. If you're running 8" slicks and doing 1/4 miles often then it'll be harder on all of your transmission than if you had, for instance, r888s on 6x13".

ITs more of a case of how its used. I've snapped a standard halfshaft with 120bhp and killed a diff with 180bhp. Give them a hard time and they break.
Plenty of mk2 escorts running 300bhp+ YB's on standard axles