I have the R9 290X custom cooled model, and I can tell you they don't OC too well, Currently my only successful max OC was 1145Mhz on core clock, once I do 1150Mhz most games crash cue to OC driver software overclock problems, so 1145Mhz will be your best bet to OC to max.
P.S. I have a Gigabyte Windforce 3X R9 290X 4GB OC for reference, not those terrible stock cooled models.