I have never tried any mirror engraving with our 2.8W J-Tech laser, but with the 2W, the feedrate was set a 26IPM with a .007 Pixel Resolution. That Angel engraving was 10" X 8" and it took 7 hours 20 minutes to complete.
You will have to experiment with the settings when engraving mirrors because the back coating from different manufacturers burns easier then others. Make sure you flip (mirror) the image first and reverse the min/max power settings in PicLaser. More laser power burns white on the front and less power looks black (reflective).