Hmmm, I don't think it's a question of extension tubes, there's something wrong with the camera or lens. Mine achieves 1:1 at about 30cm from the subject (using a 7D)...and can focus a bit closer tack sharp. Same subjects:I am confused. The spec says the minimum focusing distance is 30 cm with a magnification of 1x. So can you state the problem again?
Less than 30% crop on both pics.
My point is that even without extension tubes farhanajahan's lens/camera combo should be performing better, so there must be something wrong with the lens or camera...