To improve the COP you need a gas with a high ratio of specific heats and low Prandtl numbers. The Prandtl number is the ratio of viscosity to thermal diffusivity. Helium works fairly well for this, though adding xenon, argon or krypton improves it even further.

Really cool to see a company getting this in the market. I'm surprised though to see a standing-wave device instead of a traveling-wave device.