That doesn't help much, for two reasons:
1) Most games already use the highest process&thread prioritiy available
2) The thread becomes active between intervals of the system timer - you can only hope that it becomes active sooner than later.
I still say that the waitable timers are just software timers backed by the system timer
No, no, and no.
1) Only games made by incompetent programmers use the highest process and thread priorities available. Microsoft explicitly discourages from doing so, as it will seriously interfere with system operability.
2) A thread with a boosted priority will in any case be scheduled before any other thread with the same priority (without boost) is scheduled, and it will in any case prevent such a thread from entering its second quantum. On Win8 onwards, it will interrupt a thread in the middle of a quantum.
3) While that is partly true -- historically --, it is not true for Windows 7 any more, and even less so for Windows 8, which like Linux 3.10 uses a "tickless" kernel ("mostly tickless" is a more accurate naming). A waitable timer schedules a hardware timer, which fires an interrupt when the time is up. It then makes the thread ready and boosts the thread priority, and reschedules. Sleep simply sets the thread to ready when the scheduler next runs, which still happens periodically in a "tickless" kernel (but a lot less often).