I generally find clutch packs best in rwd's, with power side locking from 20-50% in high power cars and more in the low power things. Brake side locking depends on the track, anywhere from 40-80% usually.
Locked difs require too much compromise in the set imho. They understeer on entry at low speed and oversteer at high speed - generally the opposite of what I'd want. I see no improvement in corner exit speed with a locker - you have to reduce throttle instead of keeping it open and bleeding off any excess power through the inside wheel.
Even in the fwd's I've been moving to clutch pack lately - I find a locker provides too much variation in turning between power off and power on. Power off and they understeer like mad, power on and they oversteer like mad. Pretty much the opposite of 'normal' fwd behaviour. 80% power side on the clutch pack means I can pump the rear tyres up, set the suspension and steering up so I have fast, controllable turn in without throttle, and getting on the throttle just tightens the line a little bit, rather than madly wagging the tail as with a locker.