There will be an upper limit for the frequency at which the modules can run. If it's greater than 1000, then you're in luck. If not, you'll never reach 1000, no mater the timings.
Even though, as you said, 5/800 ~ 6/1000, it doesn't mean they can actually run at 1000MHz. Even if, let's say, you set the timings to 10/1000, which would be worse that 5/800, they will still not run.