Yes he's the most effective slot receiver in the history of the game, but THIS year, he's been just as effective on the outside. He's had several long gains after lining up on the outside. (the Revis catch, he back shoulder catch, etc) Not only that, he's turned into a red zone THREAT. Only Megatron has more WR TDs than Wes
However IMHO, the most interesting stat about Welker is his DURABILITY. Given his size its almost incomprehensible to believe that since 2004 Welker has missed only THREE games due to injury! That's even more amazing given the devastating ACL/MCL he had in 2009. I can't think of a single guy his size who can match that kind of Durability.
The most interesting tidbit Irvin gave, which was surprisingly insightful given the source, was his comment on Welker changing the entire perception of the slot receiver Rather than just being a short yardage, middle of the field, secondary receiver, the Pats and Welker have made the slot receiver, the primary guy.
As to who you would pick if you were starting a WR corp, I don't think you can pick Wes over the likes of Fitzgerald, CJohnson, or AJohnson, not if you are thinking long term. However if you are asking who is the most dominant WR to this point THIS year....then it HAS to be Wes Welker