Yes there is a performance difference between 4Gb sticks and 2Gb sticks; the 4 Gb parts have much higher input capacitance values, which loads the signals. With all other things being equal the 4Gb stick must run slower. When you put in twice as many 2Gb sticks the loads add up and so you run slower with 6 sticks as compared to 3. This is why not to go to 24G if you don't need it.
There may well be a good reason why there are no 12Gb kits on the QVL. The initial parameters come from the SPD data contained in the RAM you install. IF those parameters result in a configuration that will not POST then you don't have an opportunity to go into the BIOS and change them. You’re sort of done.
Well sort of done you can swap the RAM out to something that will POST and adjust the parameters then switch the RAM back and get it to run, but it is a hassle. Been there done that.
With Triple channel all 3 of the RAM’s need to have matched timing because the DRAM controller is going to use the same timing for all three parts. The kits are suppose be matched timing.
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