Hi John,
I mostly use a focal reducer as to speed things up and get a wider view. For example, I use a Canon 450D with a Meade 10" GPS, Celestron 6.3 reducer/flattener. With the above rig at 6.3, I can fit
much of the Orion nebula in, but at F10, zoomed in a lot more. Other nebula are quite large and some way larger than what 6.3 can see. Don't get me wrong though, if going really deep,
(well as deep as the scope will allow), and you have good tracking/guiding and an EQ mount, then F10 all the way.
Another thing to consider if you were to use a focal reducer, is there needs to be the correct distance between the focal reducer and the sensor in your camera, to work at the rated F6.3. On my scope the distance is about 105mm, not sure on yours, should be similar, but as you have the flat field ACF, ensure you have the best reducer. There are a few to choose from, some better than others as usual. But hey, you can always crop your images!
If you are using a DSLR, then the sensor gets hotter the longer the exposure so faster is better. If you have a good CCD, then your laughing either way.
If you don't mind me asking, what sort of camera/mount/guiding rig do you have?
Cheers,
Damien.