Certainly worth giving it a try, but don't expect any miraculous results out of the box. The really good thing about Hawaiki is that Simon Ubsdell (the creator of the plugin) is a first-class compositing expert and he is very responsive to feedback and questions. Once you learn all the ins and out of the built-in keyer in FCP X, you will see a dramatic improvement in your keys even with that keyer.

I have found it better than Primatte and Keylight, but there will always be a steep learning curve with any of these keyers if you want to get maximum results with poorly recorded greenscreen footage. This plug-in includes many advanced controls for matte density, edge control, and even garbage-matting and secondary mask tracking. I have had very good results with Hawaiki keyer ( /keyer.html) especially when dealing with transparent foregrounds (glass, glasses, silk dresses.) and with thin hair. That's a tough one to answer because it all depends on what particular issues these clips have. Both at the keying stage as at the rendering stage. For extremely difficult keys, Primatte will produce better results than the built-in keyer in FCP X but at a very significant time-cost. Primatte is old technology in a new dress (I have used Ultimatte hardware in my linear days), and the underlying rendering engine does not seem up to par with modern workflows. I have the same poor results in Premiere. I don't think it's a matter of RedGiant favoring any kind of software. That may be related to your external GPU. However, I had no glitches or graphical issues in the exported file. That's a huge difference, especially considering that the built-in keyer produced a similar quality key in much less time than the Primatte Keyer. With the built-in keyer, the project exported in 1'06', with the Primatte keyer it took 18'46'.

As a test, I exported a 1-minute project out of FCP X 10.4.4 on a recent MacBookPro (no external GPU). I agree that Primatte renders are terribly slow.

Red giant keyer