Because I’m flying blind, help with a couple of people on FCE but don’t own it, I would say the issue with having to render 1920x was because the timeline was set up for 1440x. 1440x is a HDV thin raster, I would never recommend transcoding true 1920x to that (unless you really have to). The computer actually has to do a lot more work to convert each frame as it plays to display the video correctly.
Anyway AIC is available on FCE and it recommended for the highest quality for FCE, everything else a lot more compressed. Once this step is done you have to match your sequence to the footage, Codec, frame size, frame rate and you should be okay.
FCE is a semi-professional app that is geared towards a very specific workflow for very specific camera formats. Once your footage is converted to what the app can handle with have a much higher end editing tool vs. iMovie.