HEPA can filter sub-0.3-micron particles, and the effectiveness trends to be even higher than its performance at 0.3 microns.

Although it's not certificated, but it's often true from a practical standpoint, due to the physics of mechanical filtration in general. The reason of using 0.3-micron in certification, is because it's the point where HEPA filters are the least effective. 0.3 um is the most difficult size to filter. Effectiveness goes up at both ends.

The effectiveness of HEPA filters trend to follow a U-shaped curve [0]. Below 0.3 um, particles are captured by the diffusion mechanism. Above 0.3 um, particles are captured by the interception and impaction mechanism. Around 0.3 um, neither mechanism is effective, and efficiency reaches a minimum.

[0] https://en.wikipedia.org/wiki/File:Filteration_Collection_Me...