Hi,
Here's my experience with my FD3s RX-7 and using a mapped water injection.
I have the injection just after the intercooler, it is currently set to come on at about 6psi and is mapped to run about 30psi (water pressure) at 2000 rpm and 90psi at just over 5000 rpm. I can't remember what nozzel I'm running at the moment though.
I set these values based on winding it up untill I got miss fires and them wound it back a little (nice and scientific).
I have a 1 litre water tank, and go through it about once every 3 weeks with general driving. Typically three or four short WOT runs each day.
When on the race track, I go through 1 litre in approx 12 to 15 minutes full on racing.
Looking at manifold temp, there is little noticable improvement with short WOT runs, this is because the stock sensor is slow reacting and gets heat soak from the plenum chamber. It's pretty usless really and I'm considering moving it to the elbow. On the track if I don't have WI turned on I get heat soak after two laps and the power drops off. With WI turned on I get a constant (about 45 deg C) manifold temp, and repeatable power for every lap.
I'm running an ARC intercooler that is only slightly bigger than stock, plus I'm running stock turbos at 1 to 1.1 bar, so I'm asking a lot of the system and generating a lot of heat. The WI allows me to run at the higher temps while also leaning out (currently between 11.8 to 12:1 AFR, but I plan to lean out some more yet).
My last dyno was 334HP ATW, at 1.1 bar boost, I'm running stock fuel injectors peaking at about 93% duty. With some fine tuning I'll get a little more out of it, but I suspect not much more
Cheers
Stephen