Have you considered building a "Spindle Speeder"?
I built a 1" OD belt driven spindle cartridge based on a 10mm shank ER11 Collet chuck, a bit of ally bar and 3 bearings rated for 30000rpm, it mounts on one side of the head and a large pulley on the main spindle gives me around 7:1 speed increase allowing use of very small end mills and engraving cutters at up to 28800rpm. Spindle & pulley cost was around £25 plus a little of my time.
Regards,
Nick