I think there are pros and cons to both and you've brought up an interesting point mentioning why probably RPMs alone is not a good choice to base the activation point on. What I am thinking about here though is very similar, namely that you may reach reach full boost and/or your boost activation point very early on in the rev range maybe as low as 3000-4000 rpms but your compressor might only reach the end of it's efficiency range at 7500 rpms.
I think you'd be hard pushed to get the water injection to start just prior to the compressors efficiency threshold in a boost based system given the above. NOW as you pointed out, you still have the issue that rpms in different gears will also equate most likely to very different boost pressures (think cruising on the freeway and accelerating away in second gear).
Airflow may be the key and maybe a MAF activated activation point based on Hz would be the answer. The compressor reaches the efficiency threshhold when it is unable to flow anymore air. Since that can be affected by boost and rpms perhaps finding a way to meter airflow and activate the injection as the threshhold is reached would allow this injection method to work irrespective of boost and rpms based purely upon load and when the airflow limits of the turbo are reached?
Paul.
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