Hmm, right something is strange here... GPU load shows 0%
Adapter 0 - AMD Radeon HD 7900 Series
Core (MHz) Memory (MHz)
Current Clocks : 300 150
Current Peak : 1010 1375
Configurable Peak Range : [300-1125] [150-1575]
GPU load : 0%
and the performance is terrible...
$ ./a.out
steps per 10 secs : 1448
steps per 10 secs : 1465
steps per 10 secs : 1468
steps per 10 secs : 1465
Anyway, there is a problem in your loop also. You are not waiting for kernel execution to finish before running the enqueueread? I get 50% better performance if I put a clfinish between enqueue kernel and enqueue read statements. But that is not very efficient... (on 7970, it now uses 50% of the card with clfinish, you should find a better solution 
...)
on the other hand, if I put clFinish on 5870, there is no difference in execution....
Yes, when you enqueue a kernel, the host program will continue and run the readbuffer command (which will try to read data from where your kernel is working on in a blocked fashion). Because the enqueue kernel command is not blocking. You should use events to keep track of kernel execution and try not to read/write to memory areas which are used by the kernel while it is executing (obviously). I think I am right, but double check from the manual 
