How much power gain can you get from a incooler?

Simply, not much! An intercooler is used more for a support system for other performance parts, such as a turbo, supercharger, or nitrous oxide (NOS). When combined with turbos, etc it allows them to run more efficiently and produce more hp. It's kinda like the foundation for a house, just a foundation does you no good, you gotta have other stuff to make it useful.