Is that windows or linux? My 780 gets 3.77khash/s and sometimes higher with only 3gb memory. On linux that it
I found it best to run a stock config and check how much memory was used. Then up it till its close to full and compare the rates. I believe my yacoin is 20x1 right now
Win7 x64, 332.21, memory usage 2360MB from 6144MB
GTX 780 hits 4.4khash/s

post please parameters, gpu Mhz and build..incredible :-)