Waning Quarter Moon: Comparing the Rebel XT and T5i

I made this image of the waning quarter moon (47.3% illumination) with 26 individual frames taken on January 24, 2013 at 5:45am. I converted RAW files to cropped TIF tiles and stacked them in Registax to reduce noise. I finished it off with a High Pass overlay layer in Photoshop to bring out some highlights. 

26 frames each at ISO 100, 300mm, f/11, 1/40 sec

While I'm happy that I figured out how to take sharp moon photos with a single autofocus point using the T5i, I'm not convinced my moons are sharper than with the Rebel XT, only bigger. Here is a comparison below using similar moon phases...

Comparison of moon photos using the Rebel XT and T5i

Both of these photos above were taken with the same Canon EF 75-300 III F4-5.6 telephoto lens, and now I'm wondering if that's the limiting factor in my moon photo sharpness? Maybe this is as sharp as this glass gets, and more megapixels will make the moon larger but not necessarily be able to scale up the sharpness?