I haven't got much to compare it against, and the 1.4TC is almost permanently attached, making it a 420mm f5.6. My only other lens, which I no longer have was the nikon 200-500mm f5.6, and I'm not sure I noticed much difference in AF or indeed optical performance. BUT I never did any direct comparisons. Originally used with a D500, but. now with a Z8.
The big advantage of the 300+1.4 is its close focussing (important for Dragonflies) With the TC, it gives X0.34 magnification from 1.4m. Not much can compete with that.
The Nikon 100-400mm is better at x0.38, but because it gets this magnification by changing focal length, you are at a working distance of less than 1 metre. The 400mm f4.5 Z lens only gives you x0.16 magnification at its closest focussing.
With the FTZ adapter, the 300+1.4 combo is about the same weight ( 1446g for the 300mm combo vs 1435g for the 100-400) and physically smaller than the 100-400.
I've rarely used it without the TC, as 300mm is usually just too short for me. But overall I like the lens a lot and it seems to work well, and It's useful to know I can take the TC off and get something a bit faster, smaller and lighter, if needed.
Without the TC, it certainly seems to focus faster.