SO you get an inch and you take a mile?
how many humans can react in the millisecond time range ?
how many games *NEED* that level of high detail timing ?
now this is just a wild guess but I bet that most humans playing most games are just fine with n times a second level of timing.
as far as I have seen DX does a good job for most all games I have seen.
Heck to get a solid millisecond timing on I/O you would need to design a whole system to make sure that for example a Disk IO did not lock a thread that in turn created a lag on delivering a key click to some app. that might not be a great way of stating that but just look at the real-time systems and know that windows is not *real-time* never has been. and with XNA you add to that .net and GC and all that....