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I trust from your response that my figures are incorrect...?

With a bit more digging I've found 1720x974 LV res for the 550D, is this correct?
I assume 1056x704 is when the camera is in standby mode.

Would it still not be beneficial to record the native size?

Instead of artificially upscaling the 1056x704 LiveView buffer to 1920x1080, it makes more sense to just record the native sensor resolution to SD Card & do all upscaling in post.

At less than half the resolution of 1080P, recording in the "native video resolution" would enable us to record for twice as long or twice the bit-rate.

Better still... dump buffer that is fed to the JPeg encoder and bypass all of this. Its major work though, more than I know how to do. Although since we can hijack functions now its technically possible.

2nd one my card could definitely do... but there is no MJPEG codec, it would have to be made from scratch.

How about just a series of Jpeg images written to SDCard? (avoiding the need to create an MJPEG codec from scratch)
Perhaps a new Folder is created for each "Record", it's a simple matter to convert image sequences to .MOV files - plus would also get around the 4GB file size limit of FAT on older camera's.

Perhaps the JPEG compressor needs to be adjusted before we'll see improved quality further down the chain...?

How about bypassing the middle man altogether & writing MJPEG .MOV files direct to SD Card? (like the older Canon Powershots used to do).
At low compression ratios, 3:1, the JPEG encoder still gives very good results, supports 4:2:2 colour space and is easier to edit - not to mention bypassing the double compression.

Uncompressed 8-bit YUV 1920x1080@25 FPS is around 103 MB/sec, MJPEG at 7:1 would give about 15MB/sec or 120 mbit/sec.

My logic is, as we can now do "low frame rate" h.264 movies, (like 1 FPS), then we don't need to work in Real Time.
So yes, we have say 25-30 YUV images on the SDCARD, then have a script to load them into VRAM, one by one.

I would imagine writing this 1 sec test .MOV may take a number of minutes to complete.

The test YUV files needn't be actual real life footage, it could be 1 sec of rendered 3D animation, with lots of detail to test the encoder performance.

Food for thought...

Unfortunately no.  It uses metrics (PSNR, SSIM etc etc) to determine quality.  Good in theory, but you would never be able to record exact scenes.  Any slight deviation would effect the results.

Also, you need a recording to base off.  There is no "perfect" copy to base the experiments off.

Is there a way to load in a RAW file off SDCARD into RAM, pass this through the encoder at various compression settings, then measure the resulting difference between these clips with the MSU tool? This would give a repeatable starting point.

1% - Any chance of another unified build? I'd like to do some testing on a 550D.

Would this tool help?

MSU Video Quality Measurement Tool (VQMT) is a program for objective video quality assessment. It provides functionality for both full-reference (two videos are examined) and single-reference (one video is analyzed) comparisons.

http://compression.ru/video/quality_measure/video_measurement_tool_en.html
There's even a free version available.

If the camera was locked off on a Tripod & the same scene captured at various bit rates, this may work very well.

Shoots a series of 1080P HQ Jpeg pictures at 25 FPS, (taken directly from the Live view buffer). Images sequences are easy to convert to .mov, easy to playback on most computers & bypass the 4 GB limits, also less complicated than trying to create a MJPEG movie mode.