Roy, what I was really getting at in that thread is that IQ goes up the larger the area of a sensor you use, in physical mm. This is why IQ from all 1.6X crop bodies is broadly similar and the first major leap in IQ does not materialise until you go up a scale to 1.3X croppers and then again to full frame sensors. Of course, you need longer glass too, in order to project a correspondingly larger image.
When you crop a 50D image to 800x533, for example, that represents a smaller part of the total sensor area (2.8%) compared with an 800x533 crop from a 40D (4.2%). There is no good reason to expect a 100% crop from a 50D to look as good as a 100% crop from a 40D : less sensor area = less light captured = lower IQ. Indeed, for any crop based on the same number of pixels the 50D stands no chance of matching the 40D. To make the comparison fair you need 50% more pixels in the 50D crop in order to represent an equivalent surface area of the sensor.
This means you should not automatically assume that you can simply to crop harder/tighter just because you have a 50D. In good conditions, with good light, good glass, good technique, accurate AF, no blur, no shake then you probably can crop that much more and get away with it. If one or more of those conditions is not optimum then you may not be able to crop tighter with the 50D, without seeing IQ fall below an acceptable level. That does not mean the IQ from the 50D will be worse than the 40D, just that in less favourable conditions it will not be better.
Feed the 50D sensor a sharp image with plenty of light and it will do a great job of recording that image in finer detail than the 40D can ever manage. Certainly features like AF microadjustment, higher resolution preview, higher resolution LCD and improved Live View AF should help you get there. But, feed the 50D with a poor image, or insufficient light, and it will record those limitations, more faithfully than the 40D can manage. The only way to conceal those flaws is not to crop harder/tighter than you would have done with the 40D.