I've just done a (non-scientific) test.
I arranged some objects, lit them with a LED panel and using a 600D on a fluid head tripod, with a sandisk 95MB/sec SD card and using ISO160, 1/50th, f6.4 i shot some video. I used a neutral picture profile with in-camera sharpening turned down to minimum.
Firstly here is a screenshot from the scene i shot, (apologies I just gathered some objects together quickly to use)

Firstly I shot a video panning slowly from right to left (about a six second pan) using ML 2.3 and CBR x1.8. The resulting video was an average bitrate of 61678kbps
Then using 1%'s version of ML, I shot another video using the exact same conditions and panning at a very similar speed, but at All-I and an average bitrate of 103320 kbps.
On playback afterwards, on a 23" PC monitor (frustratingly) I could not see any visual difference between the two clips.
I then did a screen capture of a mid point frame from each video and in photoshop zoomed to 300% and compared a similar area from each video, to see if closer scrutiny would reveal a difference.
The ML2.3 CBR x1.8 is on the left, the 1% ML is on the right. I can't really see much or any appreciable difference.

I'm really puzzled why cranking the bitrate up and making the capture All-I appears to make no perceivable viewing difference to the video quality...
NB; I'm not saying that in other lighting conditions or scenes there might not be benefits in having a higher bitrate or All-I, but I'm struggling to see how to improve 600D video for 'general' usage.
Is it perhaps that the (less than 1080HD) resolution used internally within the 600D before up-scaling to 1080 is the quality bottleneck....or is it that the 600D's H264 encoder is at or near it's quality limit and not much more can be squeezed out of it.
Any ideas?
Thanks,