bill1024
I have the EVGA FTW3 socket 1366 X58 and I am running two 8gb sticks of G.Skill 1866 with no problem at all.
They don't list it as compatible because there were no 8gb sticks for desk tops back then.
I would be shocked if your board did not support it.

The main reason it's not listed as compatible is because the vast majority of cases of people using 8GB sticks in X58's result in no POST, missing memory, or reduced speed when it does POST.  While a handful of people are able to get it to work, most cannot; those that do often cannot attain rated specs.  The CPU's are simply not designed to handle 8GB sticks with any degree of reliability.  
 
It's true that most memory doesn't make it to a compatibility list because it was released much later or the memory simply wasn't tested; this is not one of those cases.
 
Frankly, I'd just get a set of 3 sticks at 4GB apiece or go all 6 for 24GB.  The only reason those 8GB x 2 are popular, comparatively, is because most of the people buying those kits are using the mainstream line of Intel desktop boards (e.g. Z87, Z97, etc.), which only use dual-channel memory bandwidth and use CPU's that support 8GB or higher sticks.