If the object is at fixed distance, has fixed size (and is moving at constant angular speed) and fits frame well: no, you cannot get better photograph of it with focal length reducer.
Focal length reducer has three effects:
- it makes image brighter
- it reduces the number of pixels which object occupies
- if it contains glass it looses some small amount of light
1 is totally negated by 2 (and 3). Motion blur remains the same in relation to object size, you do get visibly less motion blur but it becomes just the same if you crop the photograph to frame obtained without reducer.
You can only get a better photograph of fixed size & distance object if you increase the entrance pupil, or prolong the exposure (with motorized mount).
If the object is larger than the frame with current setup, you will benefit from reducer for sure, but not if the object fits the frame well already.
Chris suggested that you could try stacking of multiple images to either compensate for noise in underexposed images to maintain the maximum usable exposure time or to improve good exposures even more - if the object is considerably smaller than the sensor. Removing noise improves percepted resolution as well. What you could do is: take a series of exposures withing tolerable exposure time and then merge them with some shifting using visible anchor points.
However, I haven't practiced it and I cannot give good software recommendation. It sure can be done manually in GIMP though.