you likely wont need the coma corrector on the DSI as its chip will not likely extend into the area of the light path that is affected by coma... I know that if I cropped 10% off the left and right sides of images taken with the QHY8 in my refractor, the field appears very flat indeed.. the DSI chip is quite a bit smaller than the QHY8, so you should be fine...
In the SCT you could use an F/3.3 reducer to get a decent field of view, and then you've also got quite a bit of aperture sucking down quite a lot of light... should make for some good photos..
Cheers.
Alex.