I am a non believer in all those additives, Take your injector question, when an injector fails they general get a leaking nozzle and poor atomisation of the fuel. The dripping nozzle is caused by a worn seat (very simular to a valve seat in the head of the engine), the only way to fix this is to regrind the seat to create a perfect seal again. Poor atomisation is caused by low injector crack off pressure (this is the pressure created by the fuel pump to crack open the seat to allow fuel to flow), the fix for this is to wind more pressure onto the spring that holds the seat shut. So there is no way any additive is going to make an ounce of difference to either of these common problem with an injestor. My recommendation for bad injectors is to fit new tips and have the pressures reset. I can give you a simular explanation to any where you would like to put an additive to why I would never use them.
cheers
blaze