Your 50d has 15 mega pixels. That's pretty darn good.. and also a very good camera. my shots from the beginning of this thread were with a 20d at around 12 mp and not raw..
When it comes to cropping, it's all about useable information. What I mean by that is that the camera records information. The more of it you have and the more of it that is sharp and in focus, the better the image will look cropped. Doesn't matter which lens but obviously, the sharper the lense, the better the image will be. And that canon 100m macro is an excellent lens. No matter which lens, the camera will take in the most information when it is set to raw.
One would get the best, closest images from a subject that is higher up or nearer to your lens..
The sensor size of a camera only increase the length of a lense in the same way a 'digital' zoom does. Again, the bigger the sensor, the more information it can take in and thus produce clearer cropped images, so it's a false increase because it is still only extrapolating from the original information obtained when the picture was taken.t.. not sure that makes sense.. anyways not really important..
You have excellent tools with that lens and camera.
Do you use a viewing box when shooting top down?
Also, does the LCD screen on the back of your camera swivel?
And can you use the LCD screen as the viewfinder?