Good question.. would probably be a good one to post up at modularfords in the KB section. I believe the 9psi kit uses a different pulley than the 6psi kit does.. (seems quite obvious now that I re-read that.) In any case, I'd post that at MF and see what they tell ya. Good luck!
Potentially yes you could use that but I'd really only consider it a band-aid and not a real solution. Keep in mind if you are running 9-10 psi and the injection fails it is going to cost you a new motor. A few guys on here have been running high boost and the injection failed for one reason or another when it was even hooked up correctly and the engine blew. The intercooler on the otherhand is a much more reliable setup and less prone to malfunction and gernading your engine.
I've heard of a couple of the 5.0L KB guys using them at the track, but not on a daily basis. Like Sid3ways stated, I don't put enough faith in them to consider them reliable enough for daily driving usage. They fall under the same category as electric waterpumps for me.
I was just thinking I could get the 6psi kit and run that on the street and when I went to the track change the pully and use the injection. It sounded like a cheaper solution and I've been hearing that the water/meth injection is better for the boost than the intercooler.