Since an official response time spec is not available anymore, what other response related spec should I check if I’m comparing your monitors with models from other brands?

There is always a trade off between overdriving the panel to make it faster which results in bad display quality. XL series fully leverage the panel fast response, but also considering an optimized setting for the overall experience. It is suggested that you can check other user's sharing on website, or test it by yourself in person which would be ideal.