exporting from final cut pro x to after effects and viceversa

Bandwidth is only part of the total equation, which takes in to account the encoding method. Long-GOP means 8-12 frames of data have to be read to play back each single frame. Lots of CPU overhead.

(Warning, this is a long yawn of a yarn.. I'm trying to explain this from back when I could just be told why before I understood the technical reasons.)

At an Apple presentation I went to in 2007 when prores was introduced in Final Cut Studio 2, they explained that camera formats should be different from editing formats. At that time there was a new camera codec coming out every other month. HDV, P2 DVCPROHD, Red was just coming out. Now that FCP X is mostly GPU based, it can crunch through a whole lot of compression/decompression. FCP 7 and back could not.

The more perceived image quality that could be crammed on flash memory or 1080 HD video on a DV tape, made it really tough on the processors back then to decompress the video.

Apple decided that keeping up with supporting every camera acquisition format was impractical. They created prores to unify mixed format workflow which was FCP 6's main new feature. Prores had major advantages, relatively small file sizes, CPU efficient, and no loss in image quality between render generations.

Before prores, I was either using the MicroCosm lossless codec from Digital Anarchy or uncompressed, Apple Animation was ancient by then. Neither of these were very practical but I could squeeze every last bit of color information during color correction or VFX work in After Effects. It was the only way to avoid a generation loss when the footage was rendered. When I would try to color correct DV in FCP, it would add the 'stair stepping' blocky effect on anything red in color. If I transcoded to uncompressed, it would minimize that because it would 'fill in' some of the missing color information. The same goes with H.264 to prores now.

(When I used prores for the first time, the DV footage I transcoded actually looked much better, that blew my mind.)

DV, HDV, DVCPRO, and now H.264, AVCHD, (even XDCAM) are lossy codecs. Prores is lossless, so it can survive a few round trips through After Effects without coming out the other end worse for wear.

As Ben said, bitrate and resolution isn't everything, there is a lot going on under the hood.