Tom,
First, you can record progressive on the PIX directly from the 1080i output of a camera. Set the scaler to record 1080p25.
Second, the ProResLT codec will have a little more noise than regular ProRes. Try a similar test using ProRes, I think your results will be better.
If you have more questions, come join us on our support forum: forums.sounddevices.com.
—Matt Mayer
Applications Engineer
Sound Devices, LLC
—Matt