Yes, you read right. I experimented with my 5d2 and my 300 2.8IS tonight. Earlier, I tried the 5d2 with a 1.4x and also with the 2x teleconvertors, but had not tried them stacked before. With some beautiful lighting, I decided to give it a try tonight.
The lens combo was certainly slow to focus. Painfully slow at times. It would hesitate and kind of wobble like IS was on at times, but then it would lock on to target and it actually worked well enough. I am not about to advocate trying this on some fast moving action when you absolutely, positively have to get the shot like your mortgage payment counts on it, but I will say it might allow you to get some decent shots of slow moving stuff a football field away.
