I have found that the DIMM I got (2x4Gb of Kingston Hyper X 1866Mhz). Is not on the qualified vendor list of my motherboard's manual. The motherboard is p8z77v deluxe. It can apparently use DIMM at higher speeds. And other amounts of Gb, there is a Kingston 1866Mhz in the list but in the size of 3Gb, 6Gb, or 12Gb.
As I can either upgrade or downgrade. I am a bit unsure on what action is now the best to take?
Dont worry about qualified vendor list in manual. Actually all memory brand supported in mobo. All model are not listed in manual.
Keep what you got. Hyper X is Kingston's product name(decent one).
No memory are factory overclocked. Actually memory cant be factory overclock. Because they have no bios in them. All memory are made as they are rated.
While memory doesn't have a BIOS, it does have an SPD chip containing a description of the memory, including the timing parameters of its RAM chips, which the motherboard BIOS will use to adjust the timings. Many SPD chips also contain an extra set of more aggressive timing parameters, the XMS profile, that often exceeds the speed parameters of any RAM chips available from manufacturers like Micron and Samsung, that is, it overclocks the memory. That's not to say the standard JEDEC profile in the SPD doesn't also overclock the chips since many chips are bought ungraded and are binned fairly crudely.