It's not all about cubic inches, it's about airflow. Larger bores allow larger valves which allow more airflow (at which point the port design of the heads become a limiting factor). RPM plays a role as well.
The DZ302 is pretty lame down low but is cookin with gas (for real) when she's wound up. Of course, that's a big bore, short stroke engine. The 305 was pretty much an emissions/economy mill.
In the end, make sure you have fun with it. If you're still short on power, I'd find a budget friendly nitrous system (I'd be conservative with it for reliability), and try that.