A supercharger or turbocharger is going to bring the most power. You're pushing beyond that $7K budget though with a supercharger if you're paying someone else to install it for a TVS supercharger (best, because it has high power at all RPMs). A Procharger you could get it under $7K with installation, but it's power gains are progressive - the higher the RPM the more boost it produces.

There are turbo kits out there for less, but often times they aren't as well put together as a supercharger kit.

Throw in custom tuning on a dyno to get the most of any of the above, and you've blown past the budget.