Curious I can record 720p24 continuous with my SanDisk 45MB/s Card
On the 650D or 700D? Could you please check your "Shoot_Malloc" size ("Debug" tab -> "Free Memory" -> Press "Q")?
Maybe you have more than 91 Megabyte. That would explain why you are getting better results with an average SD-Card ...
I think for my T3i I am going to stick to my BM Shuttle 2 recorder and HDMI out, since I bought the recorder. It will give me close enough to RAW
Have you tried working with the RAW videos? I can get 18 seconds of 1280 X 512 (2.5:1 aspect ratio) on my 600D which is enough for many situations and gives you a 2.1 crop factor. Upscaling it to 1920 X 768 makes it look really nice, even better than H.264 in terms of sharpness and detail if you use the right debayering and upscaling method. Although the aliasing is quite bad, I had no problems with moiré so far and the dynamic range is just incredible. Of course, the uncompressed HDMI out videos are slightly better than the default H.264 videos. But the RAW videos are even nicer in my opinion. And they look more like film.
By the way, I use Lightroom 4 to edit the DNG files from RAW2DNG (Almost no sharpening to avoid aliasing) and save them as TIFF images. Then I use an AviSynth script to upscale and sharpen the videos, open them in VirtualDub and export them as AVI videos using the lossless Lagarith codec. The result looks stunning and still has all the information from the RAW files in it, since the workflow is basically lossless. So you can process it in a better color grading software than Lightroom if you want.
Here's my AviSynth script if anybody is interested:
[Spoiler]LoadPlugin(ScriptDir()+"..\Avisynth-plugins\nnedi3.dll")
# Get the Plugin here:
http://web.missouri.edu/~kes25c/nnedi3.zip# Edit the path if necessary
LoadVirtualDubPlugin(ScriptDir()+"..\VirtualDub\plugins\msu_sharpen.vdf", "MSUSmartSharpen", 0)
# Get the Plugin here:
http://www.compression.ru/video/smart_sharpen/src/smart_sharpen.zip# Edit the path if necessary
ImageSource("PATH TO YOU TIFF FILES\%d.tif", FIRST_FRAME'S_NAME, LAST_FRAME'S_NAME, FRAME_RATE, false, false, "RGB24")
# That works only if your TIFF files are named like this: 1, 2, 3, 4, [...], 678
nnedi3_rpow2(rfactor = 4)
ConvertToRGB32()
MSUSmartSharpen(3)
ConvertToRGB24()
Spline36Resize(1920, 768)
# If your aspect ratio isn't 2.5:1 you may want to change the second number

[/Spoiler]