For that I focused the camera on a strong light, started recording and then I quickly panned the camera horizontally on the tripod while varying the shutter speed. Then I looked at consecutive frames and measured the light interruption (measures.jpg) and computed the fluidity normalizing it as a percentage of the average segment length.

The results are in the attachment (MotionFlow.zip) which is also shown as a screenshot (MotionFlow.PNG). They are approximations as the values can have some variation, but will clearly show you which are the good ones and the others to avoid at all cost.

The resulting videos can also be downloaded (they are very small) from here: http://www.mediafire.com/?ezz6zroqcronq28,ova2axx3qjev573,ca6o3g4cjhx46lc

As you can see, HBR 25p easily beats all other 1080p modes with no less than 5 100% FLUID motion shutter speeds, 4 of them very useful indeed. Now it's time to perform some tests to see (subjectively of course) how much we can notice it and how high can we tolerate the interruption before the motion flow stops looking natural.

(EDIT) The ideal patch GOPs should be useful in this thread. Here they are:

SS - Frequency of new frames - Ideal patch GOP

24p & 25p HBR

1/2s - 1/13 - GOP13

1/2.5 - 1/10 - GOP10

1/3.2 - 1/8 - GOP8

1/4 - 1/7 - GOP7

1/5 - 1/5 - GOP5

1/6 - 1/4 - GOP4

1/8 - 1/4 - GOP4

1/10 - 1/3 - GOP3

1/13 - 1/2 - GOP2

1/15 - 1/2 - GOP2

1/20 - 1/2 - GOP2

SS - Frequency of new frames - Ideal patch GOP

30p HBR

1/2s - 1/16 - GOP16

1/2.5 - 1/12 - GOP12

1/3.2 - 1/10 - GOP10

1/4 - 1/8 - GOP8

1/5 - 1/6 - GOP6

1/6 - 1/5 - GOP5

1/8 - 1/4 - GOP4

1/10 - 1/3 - GOP3

1/13 - 1/3 - GOP3

1/15 - 1/2 - GOP2

1/20 - 1/2 - GOP2