sure there is a difference, roughly 30% increase in hashrate. you have to update your drivers to 14.6 or ask someone to provide you with the kernel bin files compiled by sgminer on machine running 14.6 drivers. then you can use these kernel bins on mining rig with 13.12 drivers and get almost the same hashrate as with 14.6 drivers

on nicehash with undervolted 270s (core/mem clock @ 1040/1250MHz) running linux with 14.6 beta drivers i'm getting
x11 = 2.5 mh/s
x13 = 2.0 mh/s
x15 = 1.75 mh/s

with this config
{
"pools" : [
   {
      "name" : "NiceHash_X11_Auto",
      "url" : "stratum+tcp://stratum.nicehash.com:4336",
      "user" : "BTC_Address",
      "pass" : "d=0.02;f0=0;f2=0;f3=5;f4=4;f5=0;f6=3.5;f7=0",
      "profile" : "x11"
   },
   {
      "name" : "NiceHash_X13_Auto",
      "url" : "stratum+tcp://stratum.nicehash.com:4337",
      "user" : "BTC_Address",
      "pass" : "d=0.02;f0=0;f2=0;f3=5;f4=4;f5=0;f6=3.5;f7=0",
      "profile" : "x13"
   },
   {
      "name" : "NiceHash_X15_Auto",
      "url" : "stratum+tcp://stratum.nicehash.com:4339",
      "user" : "BTC_Address",
      "pass" : "d=0.02;f0=0;f2=0;f3=5;f4=4;f5=0;f6=3.5;f7=0",
      "profile" : "x15"
   }
],      
"profiles" : [
        {
      "name" : "x11",
      "algorithm" : "darkcoin-mod",
      "xintensity" : "200",
      "gpu-engine" : "1040-1040",
      "gpu-memclock" : "1250-1250",
      "gpu-threads" : "2",
      "worksize" : "128"
        },
        {
      "name" : "x13",
      "algorithm" : "marucoin-mod",
      "xintensity" : "200",
      "gpu-engine" : "1040-1040",
      "gpu-memclock" : "1250-1250",
      "gpu-threads" : "2",
      "worksize" : "64"
        },
        {
      "name" : "x15",
      "algorithm" : "bitblock",
      "xintensity" : "200",
      "gpu-engine" : "1040-1040",
      "gpu-memclock" : "1250-1250",
      "gpu-threads" : "2",
      "worksize" : "64"
        }
],      
"default-profile" : "x11",
"api-listen" : true,
"api-allow" : "W:127.0.0.1",
"vectors" : "1",
"lookup-gap" : "2",
"shaders" : "1280",
"gpu-fan" : "0-90",
"temp-cutoff" : "95",
"temp-overheat" : "85",
"temp-target" : "80",
"auto-fan" : true,
"log" : "5",
"failover-only" : true,
"failover-switch-delay" : "30",
"no-pool-disable" : true,
"scrypt" : true,
"queue" : "1",
"scan-time" : "10",
"expiry" : "120",
"hamsi-expand-big" : "1",
"hamsi-short" : true
}

for windows "gurus" , i can share my own windows build with a sample config file that i did recently during my tests with 14.6 drivers on Windows 7 (see my previous posts)

- built with M$ Visual Studio from the latest sgminer v5_0 branch (v5_0 and v5_0-x15 have been merged recently)
- sgminer configuration file included with x11/x13/x15 profiles for nicehash x11/x13/x15 multi-algo switching and wafflepool x11/x13 as a failback (intesity, tc, etc. set for R9-280x)
- kernel binaries for x11/x13/x15 algos included as well (bin files only for tahiti), generated on win7 rig with 14.6 drivers for miners running win7 with amd 13.12 driver

1.) download the package from here:
https://www.dropbox.com/s/a3zutvnjp89c322/sgminer.dwmnr.20140701.zip
2.) unpack it to Your "Desktop" folder
3.) edit the sgminer.conf file - only enter Your BTC address
4.) simply run runme.bat

ok, thanks for reply. i did some tests, the issue is related to 14.6 drivers on Windows 7. with both 14.6 versions (beta and RC2), the gpu/mem clocks are unchangeable. the old 13.12 driver works fine with the latest nicehash sgminer build (sgminer-5.0-pre-release-2014-06-30-win32.zip).

Can somebody, who is using the latest nicehash windows build sgminer_v5 (sgminer-5.0-pre-release-2014-06-25-win32.zip) with 14.6 beta drivers , please check if the gpu-engine/gpu-memclock options are working with this build as expected ?
For me, the latest windows build, completely ignores the core/memory clock settings from config file, the GPU boost mode default values are used instead (1070/1550MHz for the given R9-280X cards). I'm unable to change the values from sgminer GPU menu (Shift+G) as well. thanks