Air filters are one of those things that can have real measurable effects. However, the measurements are often more hassle than normal people are willing to tolerate.
tl;dr: I don't think you'd be able to detect a change in performance on a Prius. I recommend that you don't bother.
Air filters have measurable properties that can be broadly lumped into:
- Air flow (more is better)
- Particulate removal (less junk is better)
- Hassle (this depends on the person)
To measure the air flow of a particular air filter, you will need two pressure gauges and fittings to punch into the air intake path before and after the filter installation. In the interest of full disclosure, even I lose interest in the experiment at this point. However, if you're willing to do the legwork, you can empirically determine which filter has the best flow for your particular car.
Measuring particulate removal is more of a Mythbusters affair. You'd need to assemble a tiny wind tunnel that can force a measured amount of particulates of the relevant size (usually sand and dust). Measure the amount of grit that you blow into the system and measure the amount that makes it through to the other side. The best filters in this category will be the most effective.
A couple of notes about that last measurement. First, you'd be essentially testing the filter to destruction. Secondly, high air flow and high particulate removal are somewhat at odds, depending on the technology of the filter. For example, a foam filter that gets its regularly scheduled oil spray can often separate out a lot of particulates using fairly loose pores.
Which brings us straight to hassle. On a previous car, I put in an AEM cold-air intake which was great in terms of freeing the intake path for that little four cylinder. However, the air filter needed to be sprayed with oil and getting to it was a major pain (it was positioned down in the right front fender). I later replaced the whole intake with a Comptech ice box (when they were still making parts for the rest of us). That moved the filter up to a more OEM-like position without losing the airflow.
On my current car, I'm doing nothing. I'm using the OEM air filter on my WRX because nobody has managed to convince me that it's worth changing filter types. I also don't want an oil soaked filter sitting in front of my turbo intake.