Yeah, your gpu-engine is really low. Should be able to push it above 1100 quite easily. Also, I don't really notice much difference at 1250 memclock versus 1500. Settings between 1250 and 1500 will reduce the hash rate. It's got something to do with the memory latency profiles at each frequency. This will also vary depending on whether you have Hynix or Elpida RAM.
I have a lower gpu OC because when I get it up around 1100 I start getting hard hangs on sgminer 4.1.0 with no errors to troubleshoot. I lowered my gpu OC and that fixed the issues.
EDIT: I meant vertminer 0.5.2
That's a shame. My MSI R9 270 non-x does 1140 no problems. Also, forget what I said about 1250 memclock for vert. It only applies to normal scrypt or X11 for me. Here are my settings with thekev miner. As far as I recall they are just the default settings listed in thekev OP. Good for 250 kh/s with a single GPU rig, but I usually back down the OC to improve the lifespan of the card. Clock settings of 1140 / 1500 are in the .bat file (also with -g 2). No longer mining vert, so the image below is only after 10 mins mining. Temperature is usually 68-69, unsure of the average WU.
"rawintensity" : "5120",
"worksize" : "256",
"lookup-gap" : "2",
"thread-concurrency" : "6336",
"temp-cutoff" : "80",
"expiry" : "30",
"log" : "5",
"queue" : "0",
"scan-time" : "1",
"device" : "0",
"shaders" : "1280",
"scrypt-vert" : true
