[Fyron]

Also, how would you propose to implement both ECM and Combat Sensors? Which gets priority? Say both have a 20% modifier. Base chance to hit is, say, 80%.

Do CS first, then ECM (the other way gets the saem answer):
.80 * 1.2 = .96
.96 * .80 = .768

With either method, the 20% bonus and 20% penalty do not cancel each other, and you are left with an overall penalty to hit, even though you have the same power of ECM and CS. You would have to very carefully calculate the values of ECM and CS to make sure that they actually cancel each other, and not end up with stupid results like getting an overall to hit penalty. Or, you have to add the .2 and -.2 to the base 1.0 modifier, which results in using additive properties again. The current SE4 system does not have any of these problems. They become more severe when you start adding even more factors to the calculation (various armors, training, racial bonuses, facility bonuses, etc.)