Maybe Sigma/Tamrom will need to pull their finger out and produce newer, better lenses that don't drop down to f/6.3 at the long end?
Maybe they will, but then they'll have to charge closer to Canon's prices, and Sigron are more concerned about the cheaper end of town.
As for how the camera meters with the f/6.3 lenses, I'm not sure, but here's an idea:
The lens tells the camera it has f/5.6, when it really has f/6.3. But the aperture is wide open at that point. If you set the aperture at f/5.6 on the camera, everything will meter properly.
But if you set the aperture at f/6.3 or larger, then the camera will change the shutter speed by 1/3 but the aperture blades won't move because it's still f/6.3.
Maybe the lens just always under-reports by 1/3 stop, so choosing f/6.3 will close the blades to f/7.1? Then the metering will always be correct, but the EXIF will be out by 1/3 of a stop.
Anyway, (afaik) noone uses OTF metering anymore for normal exposures (although in a way there is when using an ETTL flash). In a canon the exposure is decided when you press the button, then the aperture blades close and the shutter fires for the right amount of time...