A couple of years ago there was a discussion on this in the tech section. A person that tested spraying alchy/water into the intercooler got a .2 improvement, but only on dry days.
Jim DelloSandros Sprayed co-2 on his and was running in the 7's a few years backk. Of course that wasn't the only thing he was doing...
I did a search on google using DIY alchy injection as a search word (s), and found an article by a guy that injected a 50-50 alchy mist pre-intercooler, and he reported substantial gains. (forget exactly how much, nut in the order of 7-8 psi gain in boost) He said the intercooler was 'cool to the touch' after several runs.
Personally I would love to try to inject propane (or nitrous) pre-intercooler. No doubt this would produce substantial gains.
Unfortunately the propane would produce a
booom! as well, should the engine backfire through the intake. I'd hate to think what would happen to the intercooler, pipes, and turbo should this happen.
I'd still like to try it....
Honestly, if I didn't have any system at all, and had $350 to spend, I would just get a nos kit, and inject it per-intercooler. A 25 shot should give you 2-3? more psi just from the cooling, and should last quite a while on one bottle.
For octane boost, just throw some toulene in the tank...