Hi Moses
Each "bin" encompass a range of values, and "T" has gone from "rare" to "reasonbly common". "U" bins are still very rare and so far, I have not been able to obtain them from Future even with regular calling. There are a few floating around the CPF.
One might infer that the "T" bins coming out today are statistically more likely to be at the higher end of the range, but I would be cautious about generalizing this to a particular purchase.
"V" is only available in Lux V, and occassionally, you can get ahold of "W" bins as well. (usually as stars)
The exception to this is in the RED and RED / ORANGE, which made some dramatic improvements recently, and are now in a new bin setup.
Just like in the microprocessor and DRAM business, power LEDs will continue their march toward being better, apparently improving a bin letter / year or so if history is a guide.