Whether the C100's codec is up to the task still needs to be seen. There are many flavors of AVCHD, some crude, others advanced. Supposedly Canon's is very good, but we'll see soon enough.
Also all 8-bit is not the same, & much comes down to how the image is processed before it's encoded. The C300's 8-bit is great, & does not band like most others'.
The use of an external recorder will depend on your grading needs, but most of Sony's, & Panasonic's, professional users haven't seen the need. 8-bit 4:2:0 capture needs to be handled more like chrome than RAW, but I've shot green screen with it that keyed beautifully.
Supposedly Canon has been working with Atomos to ensure compatibility with the Ninja 2, but their recorder's drive trays can be a bit tricky to remove. As Morgan stated, the Pix 240 is a remarkable piece of kit, but it's also a bit heavy.