>> C8+reducer+incorrect backfocus = flatter field frames than
>> C8+correct backfocus (without reducer and field flattener)
A shorter focal length does not have to mean a larger usable FOV -- it depends on the optical designer. The primary purpose of a reducer is to get a lower f-number.
For example, on my FSQ85, the 1.01x flattener gives a substantially wider image circle than the image circle from the Takahashi 0.73x reducer. (almost full frame image circle vs the 0.73x reducer, which has a usable image circle of perhaps 11mm; although "usable" depends on the how tight you want the stars to be). See here for example of a star field using the 1.01x flattener with my ASI2600 (APS-C sensor):
http://www.w7ay.net/site/Images/FSQFlatness/1.01x_APS-C.PNG
The 0.73x produces a smaller usable FOV than the above after cropping out the wonky stars. However, it uses a factor of two shorter exposures with proper pixel scales.
As a result, at least for the FSQ85, the reducer shortens exposure time, but the 1.01x flattener gives a wider field when combined with a full frame sensor (the 0.73x reducer does not completely cover even an APS-C frame).
I have both the Celestron 0.63x reducer (the original one from Japan, bought back in the early 2000 for use with my 8" Meade back then) and the Starizona 0.63x reducer/Coma corrector. The latter gives much tighter stars, over an image circle of 27mm (I don't get that large an image circle with my 6" SCT, though; perhaps I am just more critical). Celestron claims a 24mm image circle for their reducer when you use a 105mm back focus (I also can't get clean stars over an APS-C sensor when using the Celestron reducer with my 6" SCT).
If you can't get to 105mm back focus, then you really can't use the Celestron reducer. The field won't be flat.
The Starizona 0.63x has a shorter backfocus (90.3mm) than the Celestron reducer, so it won't work for you either. The upcoming Starizona Night Owl (0.4x reducer/corrector) has an *even* shorter back focus (38mm) and even smaller image circle (16mm), but it has an extra f-stop yet in speed.
Again, a case where a reducer gives higher speed optics, but a smaller usable FOV of the sky.
By the way, Astrometry.net plate solved your JPEG image after an extraordinary long time (many minutes), with a FOV that is some 2% off of the FOV that you stated. It could be caused by the time I submitted the plate (busy period), or it could be a distorted field that causes astrometry.net to take a long time to solve.
I had made a request to ZWO to provide selectable ROI for any sensor. Right now, ASIAIR only allows a full frame sensor to download as an APS-C crop. "Virtual sensor sizes" like that for any sensor (saves time when post processing small galaxies) will allow you to also crop out the wonky stars at the edges of a frame before they are given to the plate solver, assuming that it is still larger than the minumum FOV that ASIAIR can solve.
Chen
