The issue here is how good is the price.
Most birding is done at the long end. SO on paper a 300 with a TC is almost the same and would be quite functional. Plus you can take off the TC if you need shorter. But overall a zoom is more flexible. My two primes are 200 and 300 are ƒ2.8, that's what makes my TC stacking functional. The 300 ƒ4 didn't interest me simply because of my experience using the TC and the DA*60-250. I don't really like TCs on ƒ4 or slower lenses. But others seem to do OK. I regularly use the HD DA 1.4 TC on both my primes and the DA* 60-250, and results are acceptable.
So for me it would come down to "How good a deal is it? "
Having invested in a 6 pound 300 2.8, (420 ƒ4 with the 1.4 o4 510mm ƒ4.5 with the 1.7) I'm not currently considering the 150-450, but if you don't have a 60-250, 200 and 300 primes, I'm sure it is a lot more attractive. To me, it's a great lens that would take away most of the need for 3 of my other lenses. But I'm used to what i do, and have no desire to change. I can see how it could easily be someone's one and only long telephoto, and it's a great choice for that.
So don't think of it as an expensive lens, think of it as the lens that saves you from buying the 60-250, 200 , 300 and 1.4 and 1.7 TCs lens. All the sudden it sounds like a bargain. I can totally imagine carrying the FA-J 18-35, DFA 28-105 and this lens as a K-1 expedition kit.
As long you can carry it all will be good.