The windows OS is always by Microsoft, no Asus involvement
The optimization your speaking of would most likely come from some form of Asus software or the devices software or from within the board itself
For example let's just say SSD speeds, windows can effect it a bit but, the Intel sata ports on the board, and the Intel management software would effect it more, kinda sounds confusing but I hope you get what I'm saying lol
If you have a free 8.1 upgrade I'd say try that out first as other suggested, with classic shell you may actually like it, like I was saying above it all comes down to personal preference really, unless you run something that actually requires windows 8-8.1, in which case you'd wanna stick with it
CoolerMaster HAF 932 Advanced/ Maximus VI Formula/ I7-4770K/Swiftech H320/ Corsair HX850/ G.Skill Trident X (2x8) 16gb 2400MHz/ 2x 840 EVO 120gb(Raid 0)/ WD 1TB HDD (Backup/Storage)/ EVGA GTX 1gb 560 TI/ Asus 12x bluray combo