ROI should be positionable within the frame, so you should be able to place and size the ROI as you need in order to optimize the performance of your focus throughout the frame. The ROI should not always be centered.
I think that AF would really need to be an optimized pipeline in SGP, tuned for the kind of camera. I think CCD benefits might be limited beyond simply the readout speed increase of ROI.
CMOS, however, could potentially benefit from a rethink in how AF is done. The way the normal downloads work, it currently takes ~2 seconds for SGP to read a frame even with the very high speed CMOS cameras. I know those cameras can read out a single full frame in about 0.4 seconds, and in video mode with an ROI of even 400x400 pixels they can crank out several hundred frames per second. I think it might be possible to use uncompressed ROI video and simple frame-grabbing to dramatically speed up the AF performance if you ROI brighter stars. And in the event that brighter stars are not available, with a dramatic improvement in AF performance it should be more viable to slew to a nearby bright star, AF on that, then slew back, and still be done in less time than it currently takes an AF run to complete.
With video ROI and frame grabbing, it should be possible to yank out multiple samples per focus point in seconds (or if the stars in the ROI are bright enough, less than a second), run through a 10-15 sample AF routine about a minute or less, and get back to business running the sequence as if AF barely happened.
Throw in an AF frequency curve, so that you can focus more often earlier in the evening, and you can improve things even more. The cost of doing AF in areas where there is a high temperature gradient through each night and frequent AF for the first few hours of each night is required, and possibly high frequency through lower altitude imaging is required, the cost of AF is fairly burdensome. It currently takes me several minutes every 10 minutes to run AF for the first 2-3 hours of the evening right now, which is an overhead cost of around 25-35%.