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[so they should be clean from compression and motion artifacts]

Those compression ratios are JPEG's "Highest" and "High" Quality compression standards, so they should be clean from compression and motion artifacts ;-D  Which only leaves line skipping and an 8bit color depth to worry about right? 

Hardly, you will see more blocking from jpeg compared to h264, as jpeg lacks in-loop deblocking. You will likely see more motion artifacts from jpeg as it does no sort of motion compensation. It would be better if the h264 engine could be used in place of jpeg, but it's probably impossible due to the h264 encoder's fixed input sizes.

I'd say the latter. I had CF to IDE and it would run at like ATA33 or 66 so probably won't get full speed with the translation due to overhead, etc.

CF to IDE is not really comparable, as a CF device is actually an IDE device; adapters between CF and IDE are purely electrical and don't have a controller on board. Conversely, a CF/SD adapter will have the controller on the adapter.

That said, I'm very doubtful any CF/SD adapter has a controller capable of UHS speeds. There's a reason UHS cards are expensive.

Technically speaking, that's probably the right thing to do for the Canon CMOS sensors. None of the cameras benefit from ISO > 1600, and all that is being done is lopping off one stop of highlight dynamic range at each higher ISO stop.

It should be noted that for the same bitrate, shorter GOPs yield significantly poorer compression. GOP=18 is more efficient.

If you were to record 720p with 60frames per second and the shutter fps override to 1/12 and then in post use slowmo to get normal speed but great more f stops.

can 60fps really be recorded i720 with fps override?

Just take 1/60 frames with 720p60 and do a five-frame temporal average. Easy to do in AVISynth.

I can think of a hardware limitation for the 1000D which (i'm counting on my memory now) had a mere 1.5 fps burst. Considering the same shutter and mirror assembly as the 450D, i guess that was something in the FW that prevented a better performance.

Actually no, the 1000D has 3fps burst in JPEG. It's artificially limited to 1.5fps in RAW.

I think the low end cameras have the shutter and mirror coupled together. So XXD and above cameras don't need to drop the mirror to actuate the shutter.

From the patch from http://www.magiclantern.fm/forum/index.php?topic=3085.0 :

Code Select Expand

+static char file_prefix[8] = "IMG_";
+PROP_HANDLER(PROP_FILE_PREFIX)
+{
+    snprintf(file_prefix, sizeof(file_prefix), "%s", (const char *)buf);
+}
+
+int hdr_increment_file_prefix()
+{
+    if (hdr_counter < 0) return 0; // you have poured alcohol into the CPU?
+    hdr_counter++;
+    if (hdr_counter >= 1000) return 0; // you took 1000 bracketed shots?!
+   
+    if (hdr_counter < 100) snprintf(hdr_prefix, 8, "B%02d_", hdr_counter);
+    else snprintf(hdr_prefix, 8, "B%03d", hdr_counter);
+
+    while (lens_info.job_state) msleep(100);
[b]+    prop_request_change(PROP_FILE_PREFIX, hdr_prefix, 8);[/b]
+    hdr_file_prefix_dirty = 1;
+}
The 550D doesn't support FIO_RenameFile, but from the above code, it seems like it's possible to customize the file prefix of captured images before capture.

Can the replacement of IMG_ and _MG_ with arbitrary 4-character strings be added as a feature?
This is a feature only found in the high end Canon cameras and makes file management much easier (in case of multiple non-1D / 5D3 cameras).

Not sure how this would affect movies (MVI_) or whatever else is implemented in the firmware.