Presumably, this is a continuation of the older thread.

I had an _epi32 mergesort implemented from earlier time ^(*), and so repurposed it for the given task. No doubt a parallel version would scale rather nicely, too, though this I haven't tried. The 7360x4912 pixels of sciurine menace obtained via dcraw.

$ perf stat perl rgb48.pl squirrl.dat 
[1452115604.189627] sort_and_uniq:
[         0.294368] first binning
[         0.250713] second binning
[         0.352289] merge and count
squirrl.dat == 35556508

 Performance counter stats for 'perl rgb48.pl squirrl.dat':

        937.769123 task-clock                #    1.000 CPUs utilized          
                13 context-switches          #    0.014 K/sec                  
                 0 cpu-migrations            #    0.000 K/sec                  
             55529 page-faults               #    0.059 M/sec                  
        2775248733 cycles                    #    2.959 GHz                    
        1375100650 stalled-cycles-frontend   #   49.55% frontend cycles idle   
         588508119 stalled-cycles-backend    #   21.21% backend  cycles idle   
        3397637328 instructions              #    1.22  insns per cycle        
                                             #    0.40  stalled cycles per insn
         310595954 branches                  #  331.207 M/sec                  
           9170806 branch-misses             #    2.95% of all branches        

       0.937321108 seconds time elapsed

Note, this is Lynnfield CPU without avx. Same test in a timethis loop:

timethis 10:  8 wallclock secs ( 8.33 usr +  0.00 sys =  8.33 CPU) @  1.20/s (n=10)

There are other optimized sort implementations out there. Intel IPP (Integrated Performance Primitives) has the following routines, among myriad others

IppStatus IppsSortAscend_32s_I(Ipp32s* pSrcDst, int len);
IppStatus IppsSortAscend_64f_I(Ipp64f* pSrcDst, int len);
IppStatus IppsSortRadixAscend_32u_I(Ipp32u* pSrcDst, Ipp32u* pTmp, Ipp32s len);
...

I'd expect these to provide a well-optimized solution for any Intel platform.