Is a better air filter worth it?

I have always ran with K&N filters on my stock 1.3 Micra's, just install once and that it.
The 1.0's are pretty gutless, so changing the filter (unless you go induction), probably won't give you much more power!
In theory it will give more power, but in reality it'll be probably less than 0.1hp.

It's a common misconception that changing one thing will improve the power, more power comes from more air and fuel going in. But the air flow needs to be thought of with the entire system, from air filter through to the end of the exhaust, if one (or more) part is blocking the flow then the best filter in the world wont do anything. The manifold and head design are probably your biggest problem.