because of the difference in latency there is little difference between the 2. when you go to 2100-3200 you will see an increase if your hardware fully supports the higher speed. problem is most of it doesnt. intel for instance handles 1333-1866 depending on what gen i3/i5/i7 is being used. running faster ram on these builds wont actually give you more performance apart from in synthetic benches.
ddr3 1600 cas 9 is as efficient as ddr3 1866 cas 11 with less than 1% performance difference. me i still prefer to get the lower cas ram over the higher speeds 1s i reach the maximum native memory support offered by the cpu.
As freq goes up, you are actually getting more bandwidth of DRAM that can operate per clock cycle and process data....Actual performance is a combination of freq and CL (CL is how many clock cycles it takes to perform an operation) so technically in your question the 1600/7 would perform slightly better than the CL9/1866........if it were 1600/8 and 1866/9 the 1866/9 would get the slight nod for the higher bandwidth...in general w/ performance DRAM you'll look at a step chart where performacne is given to the higher freq for the higher bandwidth