[rotrex]

I'd rather only spray into the engine. Just spray more if you need to extract more power with more boost.
By the time the effects of the external sprays turns relevant, you're off the throttle again anyhow. Best way would be to try it.
The extra effort is minimal once the system is installed.
Can you remap your ECU? Without that, you cannot inject much into the engine anyhow before power drops.