ABS conversion

It's important to point out that the early ABS was designed to work with stock suspension, 14" rims and higher profile tyres.  Tuning an ABS system critically involves these factors and major changes to them WILL degrade its performance if not adjusted to suit.  I think I can safely say that almost every early SW20 on the road today has bigger rims, lower profile tyres and/or modified suspension.  This renders any subjective opinions of the effectiveness of the ABS void.
 
Also, all ABS systems have independent sensing and processing of the speed of all four wheels, regardless of the configuration.  The number of channels refers to how many mechanical systems the unit has to reduce braking pressure.  Three channel ABS releases both rear brakes simultaneously if either locks, due to only having one hydraulic circuit split to both calipers, but still controls the two front wheels independently.  Four channel can release the rear brakes independently of each other, resulting in a minute improvement in braking performance.
 
The early SW20 ABS is 3 channel, but having a higher rear braking bias than most other cars means that it will gain more benefit from a 4 channel system than front engined cars.
 
Most opinions of ABS systems are based on the system having activated and given the driver the feeling that the car is not stopping as well as it could.  In most cases though, the conditions were abnormal and in the absence of ABS one or more wheels would have locked resulting in a potentially more dangerous situation.  Proving one way or the other is practically impossible, it's very hard to assert with any authority that in a hairy situation ABS was counter-productive, unless it's done under controlled conditions repeatedly.  Panic braking on the roads are hardly controlled conditions with unbiased observers!