If you want some real power and I mean real power add a super charger. It is a one day job that really gives you lots of low end torque; there are kits for many engines. I like these because you don,t have to monkey around with the exhaust. You may want to add an intercooler as the air gives off heat when you compress it. These are really fun for low displacement engines.
If you don't go overboard you can keep the lower end pretty much stock. Like 5lbs or less.