Once a (fairly) long time ago, I found this interesting, so I did some testing. In my case, I was less concerned about whether something was labeled L-series, but about primes vs. zooms. Given the number of elements in many (most?) zoom lenses, I'd guess the light loss is even greater than differences in glass could account for.
Due to the timeframe, I was doing the testing on Velvia -- high contrast slide film that was notoriously picky about exposure, which should have maximized visibility of exposure differences.
The results varied somewhat. In a few of the best cases, I couldn't see any real difference between the zoom and the fixed-focal-length lens at all. In one case (an ancient hyperzoom, something like 50-250) there was probably about 1/4-1/3 stop loss at the center.
Toward the corners of the frame, the differences were much larger. In fairness, however, this probably doesn't mean all that much -- to get to the same apertures, the primes were stopped down quite a ways whereas the zooms were being shot within a stop or two of wide open. Greater light falloff when the lens is wide open (or nearly so) is extremely common. Zooms are generally worse in this respect than fixed-focal-length lenses as well.
The only way that's likely to apply in the case you care about is roughly the same one: at least in some cases, an L-series lens will be faster so to get the same aperture it'll have been stopped down further, which will usually reduce the light falloff at the corners. OTOH, this is primarily an issue when shooting full-frame -- if you're shooting APS-C, it's much less of an issue.