edit Totally misunderstood the question, which was whether an L lens at the same f/ number transmits an extra stop of light by virtue of lower dispersion etc., the answer is no it won't be anything like a full stop, but there is probably a small advantage.
Our friends over at DXO mark test transmission of lenses, though only wide open, and they don't have a pair of L non L lenses at the same speed. What the results do show is that the wider the max aperture the more transmission you loose compared to the theoretical f stop. So the 50 f/1.2L is about a fifth of a stop further away from the theoretical value wide open compared to the 50 f/1.8
Since manufacturers (of SLR lenses) don't release t-stop information the data is scarce so most photographers refer only to the f stop value when they talk about lens "speed". Cine lenses are often specified by t-stop is this is more important to film as you can't (easily) change the shutter speed.
You are right that the f/ number tells you how fast a lens is. The lenses designated "L" tend to be faster, but that's because of the lower f/ number, not the L!
Case in point, the EF 100 f/2.8L is not faster than the EF 100 f/2.8 as they have the same f/ number. However the EF 35 f/1.4L is a stop faster than the EF 35 f/2, because the f/ number is smaller.
The f/ number relates to how large the aperture is in relation to the focal length. f/2 means the aperture diameter is the focal length divided by 2. Wider aperture means you let in more light and get more background blur which is generally desirable.
Being a stop faster means you let in twice as much light, which means the area of the aperture is twice as big. Hence if you square the f/ numbers and one is twice the other one, the smaller one is a stop faster.