Great guide, and can confirm that it does work!
I managed to use this method to restore my angry birds data from a Froyo phone (HTC Desire), to a Jelly Bean phone (HTC One X+). This is no easy task since you cannot use adb backup/restore on Froyo...
I am almost 100% sure that you are generating an invalid tar file.
Is there a unique attribute applied to the original tar file then? Any idea how to generate a valid tar file? :)