Using a 6.3 FR will reduce your imaging time for the same exposure by about 3 (roughly). So if that was a 9 minute shot you would get the same exposure in 3 min. You will also get a much wider field of view.
To get a better handle on this stuff Barry I'd suggest you download the CCD Calculator from
www.newastro.com. You can download it
from this page It has images that will show you how much you will capture using different setups. You may need to input your 400D details for the camera section but that is pretty straight forward.
You will get vignetting using the 6.3 reducer, but you chip is big enough that a bit of cropping won't be a problem. Once you learn to do flats though, that will help.