Hi Ben, there is already an open PR to open to the latest stable head (see linux-raspberrypi: update to latest stable head 5.15.92 by alexgg · Pull Request #1018 · balena-os/balena-raspberrypi · GitHub). Once that is merged and a new release is deployed please re-test and let us know.
In general, balena-raspberrypi follows meta-raspberrypi at the corresponding Yocto version. In this case kirkstone is using the 5.15 kernel version. The policy is usually to use the latest Long Term Support kernel.
In terms of stable branches updates, we are looking into automating recipe updates so we always use the branch head, but until that happens we have to do it manually and usually on support requests.