Okay, I think I understand how the whole lens, camera metering works. The lens tells the camera its max aperture, the camera takes a reading based on that and the light it's seeing, it's actually basing the calculation on the t stop it's seeing relative to the f stop it's given?
T stop is just a theoretical (measured in a lab??) value.
When you half press the shutter the meter is activated and the lens does nothing really, it's already wide open to allow as much light in as possible. The camera then does it's calculations based on the amount of light hitting the sensor.
When you fully press the shutter the aperture then adjusts to the required setting and the shutter moves to achieve a correct exposure.
The only time you might notice T stops is if you were using M mode and switching lenses for the same scene. For example lets say you set up a shot at f/2.8 1/60 ISO 100 and it looks perfect. You then switch lenses and set it to the exact same settings and notice the shot is under exposed slightly. No big deal you just crank up that ISO or adjust shutter speed and you're back on track!
An example where you might encounter this situation is say you're using the 24-105L and shooting at 24mm and say f/8. You decide you're not a fan of the 24-105L at the wide end so you switch to your 24L or whatever and try again. Surprise surprise it looks better now!
That's basically what this is all about. The Sigma lens would be slightly brighter and of course better. But the amount is fairly small.