I imported four clips from a Panasonic "ZS3 Camera Archive" into iMovie 09, and imported the same clips after transcoding the source clips with Handbrake64 to H.264, (x264 codec, not Apple's H.264 which has Gamma shift), 17000 kbps bit rate, MPEG-4 .mp4 container. The .mp4 clips are about 10% bigger than the source clips from the camera.
The .mp4 clips were copied straight into iMovie by the Movie Import w/o transcoding to AIC. (The AIC clips are 3x the size of the .mp4 clips).
Interesting to note is that the AIC converted library clips show terrible distortion when previewed full screen in iMovie, while the .mp4 library clips do not show any distortion in full screen preview.
In an export to AIC the resulting movie is 980MB and I cannot see any difference between the two import methods when viewed on my 27" iMac display.
The Vimeo export is H.264 (again x264 codec) CABAC with adaptive B frame, key frame every 24 frames, restricted to 4800 kbps.
(I don't know why key frame should be 24 not 15, 29 or 30. Steve Mullen in "The Ins and Outs of iMovie 09, Second Ed." said 24 is desired for Vimeo.)
The clips: Clock Face with Color Chart Colorful bird House with clouds and blue sky Flower Moving titles with several different types of computed transitions.
Also of note is the cpu usage when rendering: 4 cores approximately equally active at 4 to 5 bars, CPU reporting 45-65% busy and 38-55% idle, 1.78 GB active, 3.39GB used.
So I am glad I bought the Core i5 iMac, although these stats would say a core two processor would be just as fast but loaded heavier.
So now I have a way to import my pre-Mac AVCHD lite .m2ts files, and a workflow choice for new camera footage.