I have the similar issue with printing a Arial pt 2 from google doc, using 600 dpi is ok, 2400 dpi is making all words blurry.
It looks like 2400 dpi is converting the words (vector graphics) into pixels and then there is a blur process applied on it. This kind of rasterization process is failing.
The only other thing you could try is configuring the PDL settings in the Embedded Web Server. If this doesn't help, I would recommend contacting the Support department to speak to a product specialist: 1-800-821-2797.
The WorkCentre 6515 advertises a 1200x2400 print resolution. I cannot get it to go above 600x600 and photos look like trash, even on glossy paper (yes the paper is specified for laser printers). I've tried to adjust the Xerox driver software to "Enhanced", and to change Windows settings to 1200x1200 and 2400x2400. It always defaults back to 600x600. When printing, there is a dropdown for higher resolutions, but when I select one, it instantly goes back to 600x600. The dropdown will allow me to select a lower resolution, but not higher. I have tried from different machines with the same result. I don't expect the same quality as a professional photo printer but it should at least be decent, and for now it is not. The images are from professional photography equipment and edited by a professional so the image files are not the problem.
Is there a way to increase the print quality to a higher resolution as advertised? I can't print professional documents with a grainy photo quality. Thanks for any help you can provide.