...The "video quality" option adjusts the compression level...
could you please be more specific about this option?
The G1X (like other Canon cameras that record H.264 video) attempts to maintain a constant quality in the video rather than a constant bitrate. The bitrate (and file size) will depend on the content being recorded - using the default camera settings I've seen 1920 video recorded as low as 50MB / minute up to 250MB / minute.
CHDK allows you to override the 'quality' factor applied. The values you can enter range from 1 - 99, where 1 is the lowest quality and 99 the highest. The default camera setting is approximately equal to 84 in the CHDK override.
A setting of 99 will roughly double the average file size, a setting of 1 will be around 1/2 - 1/5th the average file size; but the actual size can vary greatly as I said depending on content.
A high speed class 10 card should be able to keep up even at quality level 99 - I use a Lexar 400x card with no problem.
The last issue is the 4GB file limit - at quality 99 you can easily reach the 4GB limit in around 8 minutes recording 1920 video.