I heard that Sigma 50-500 is better than 150-500, but i can't be sure if that is true, also i still use and love my 100-400L even i used it last time in 2008, but ofcourse if i have a budget i will go with those expensive primes.
I also recommend 100-400 or 400 f5.6L, i don't prefer to go with prime lens+extender and crop also for birding unless it is one of those big primes, 300 is limited and even with 1.4 or 2x it is not much use and love, i have 300 f2.8L IS and even with any of TCs i don't feel happy using the combo for birding even on a crop body, 400-600 are the best primes bare instead, i will always choose 400 5.6L bare over 300+1.4x, even 500 bare is more preferable for me over 300+2x.