This decision is quite controversial. Although a fast lens will give some extra speed and quality boost, they will still depend on how the camera performs. You won't be able to feel the full lens speed because the camera will limit it. It's a similar story with picture quality. It will probably be a little better, but not at the lens's cost.
And also, the suggested bundle follows one not quite obvious problem. Imagine you are shooting a shaggy active dog with a 135 lens f1.8. Because the dog is active and your camera is slow, it might focus not on the eyes but on the fur right near the eyes. If you're shooting at 50mm f1.4, nothing critical is likely to happen, but if you're using 135 1.8, the eyes will go into a severe blur, which will be an outright defect. Why is this so? Because the 135 1.8 has a very narrow DOF (depth of field). Small DOF + slow focus + fast dog = a lot of blur. That's why I wouldn't recommend considering such a combination.