Yep, it does seem to report increments of 1 millisecond. (If you run GetTime() in a loop just printing it to chat, the values are precisely increments of 1 millisecond.) I'm not sure the time is accurate to the millisecond though -- are all milliseconds created equal?
The other problem is that a lot of functions run much quicker than a millisecond. Lua is running about 1,000+ OnEvent/OnUpdate functions per second. That probably fans out to 5,000+ functions per second.
I'll show timing data in v1.4 though. You've helped me convince myself that it's not that big of a problem. I should just try simple timing and see what happens.