Hi Greg,
There is a known limit to the feedrate using the J Tech GRBL version 0.9g due to the stepper implementation in the main GRBL branch. There is a lot of floating point math in the stepper ISR that will cause a limit on the feedrate allowed. This is not a huge concern for using the CNC, but can be a problem when using a laser.
Sonny and I are working together on the laser implementation on the new version 1.0 of GRBL main branch. We have merged the J Tech firmware laser implementation with his main branch and it should be able to do 8000mm/min. In the meantime, you might just need to lower the power if you are hitting the maximum feedrate limitation due to the stepper math. The new version of the firmware should be available soon. Sonny is working on it now and we will be testing it shortly. It will be a big advancement for all the laser users who use GRBL and for us as it is a big task to try to update our firmware every time the main branch does something new. J Tech Photonics is proud to be an official sponsor of GRBL and we look forward to this new release. We will keep everyone posted on this upcoming release of the new firmware!
Best Regards,
Jay