Make sure you checked "Extend official overclocking limits" once you click on Afterburner's properties tab.
You can change the BIOS and set the max frequency that u want, independently from the value set by the producer.
However my cards became unstable over 2200 (I am using 1750 timings)
However my cards became unstable over 2200 (I am using 1750 timings)
Which particular cards are you speaking of?
Are you running on stock voltages/core clock?