P1, Q2, R5 ... for Cree, those are "flux bins."
What's flux and what's a bin, you say?
Flux is a generic term for brightness in the LED business.
Bins are used to categorize performance. As part of the manufacturing process there is normal variation, with some parts performing better than others. During the heyday of the Luxeon Lumileds used to bin for Forward voltage (electrical efficiency), tint, and flux. Cree bins primarily for flux; their forward voltages are pretty consistent and their tints seem tight enough that just going by CCT (Correlated Color Temperature or degrees Kelvin) is tight enough for most applications.
So a R1 is brighter than a Q2, but a R4 is brighter than a R1.
Cree has released a steady stream of differing LED chips and packages in recent years. XR-E, XP-G, XM-L, XB-D, etc all define specific packages - many of which share the same LED chip within - with different levels of performance both in terms of flux an optical performance. The XM-L is hailed as being one of the most efficient LED's on the market, but its large chip/phosphor area makes it less-ideal for reflector applications (most flashlights) since that makes it less of a point source.
There have simply been too many LED's out there to devise a neat chart. Perhaps the DoE has some chart as a result of their L-Prize (or some other semi-neutral organization).
Companies that come to mind in recent years when it comes to the evolution of LED's (not all of which are terribly important):
Lumileds (now Philips)
Nichia
Seoul Semiconductor
Osram
Lednium
Lamina Ceramics
Cree
Bridgelux
Luminus Devices
Toyoda-Gosei
Edison
Stanley
Citizen