Meth injection without tuning WILL help, but only a little. It's basically like running a higher octane fuel, and you'll be cooling the air charge before it enters the engine as well. That may be good for a few HP, and it will be better than running 91 octane if your state doesn't provide 93, but just like putting 100 octane race fuel into a street tuned car, you won't really see any benefits above and beyond what it's programmed to do at the factory.
In essence, it doesn't hurt, but mostly all you'll be doing without a tune is ensuring it doesn't perform below the ideal factory performance. Whenever a tuning device comes out, you could increase the boost and timing a bit and keep the kit in.
You could always hit up Devilsown and see if he can sell you a kit, I had one on my SRT-4 back in 2004, it helped me deal with the crappy 91 octane in California and avoid having knock.
http://www.alcohol-injection.com/en/205-dart-ii