To absolve any confusion:
The way the games determine if its shiny or not is more or less this:
(PIDlow^PIDhigh)^(TID^SID)>>3 = 0 -> Shiny [Gen 3-5 origin] (return 0-8191)
(PIDlow^PIDhigh)^(TID^SID)>>4 = 0 -> Shiny [Gen VI origin] (return 0-4095)
Without the shifts, pre gen6 the XORs had to return a number 0-7. What we noticed in Gen VI is that shinies exhibited 0-15 results, thus the overall odds were doubled.
The whole shiny value stuff is a fan simplification since PIDs and Trainer IDs can't be altered by one user, only the OT can be changed (giving a new SIDTID).
I don't think anyone has bothered checking xor=16, but there shouldn't be a difference between wilds/breeds. They can always roll extra PIDs if it's not shiny (like genV shiny charm or gen45 MM, or g34 everstones... you get the point)