I don't think it really matters what size turbo is run. With boost based flow control(numbers 2-3), you only get 1 flow no matter what rpm you're at. Let's say you want 8gph at 7000rpm and 20psi. You will also get 8gph at 4000rpm and 20psi because the boost is the same, and the flow is based on boost. Let's say you have a 2 stage number 1 with 2 4gph nozzles(or a 5 and a 3, or 6 and 2, or whatever). At 4000rpm you'd get 4gph instead of 8, and at 5500rpm or whatever, the second nozzle could kick in giving the same flow. This second flow makes it so the water/fuel ratio can be more consistent throughout the rpm range, and this could help an engine perform better.
A few things to think about when deciding what kit to get..... People don't usually adjust their controllers once they're tuned and what not. This is cause cars are tuned for a given amount of injection, and changing that would warrant another tune. Because of this, having the controller be accessable isn't really necessary, and it might even be more dangerous then anything(like if a setting gets changed by mistake). Another thing to note is that the engine compartment is a hot place. It might be nice to mount a windshield washer tank and pump there, but your engine doesn't rely on windshield washer fluid to servive. I know you say to yourself, "it must be ok since this company has it like that", but some kits were made more with cost and ease of installation in mind then funtion, safety, and reliability. Just some food for thought I guess.