It's strange... But on my Pitcairn card 270X 4Gb miner can't use more than 3.5Gb of vmemory... 1 thread 1856 and 2nd 1644 (multihash 464 and 416 or 448+432) = 3500Mb. Increasig any thread without lowering other leads to OpenCL error on creating buffer for 2nd thread...