Stephane,
As far as I know, after applying the patch, the size of .xml files is increased only once (when you run the tests for the first time after installing the patch). This happens because the patch gathers more data than TestComplete does. The file size doesn't increase in subsequent runs. Do you still notice the increase after each run? If you do, tell me how much it is. Which TestComplete version do you use?
As for the update speed, how often do you run tests in the Auto-update mode? Is it possible to turn off auto-update? Is the speed acceptable after you turn it off?
You said you would like to have Visualizer images to find the cause of an error, if any. TestComplete captures images for test commands that simulate user messages and post event messages to the log. So, you can try using the Save last N events project property (double-click your project in the Project Explorer to open the project editor, then go to Properties -> Playback). This setting specifies the number of events to be saved in the log before an error message is posted. Assign some value, say 5 or 10, to the property. This number of images should be enough to understand what went wrong, and the images will not occupy much space on your hard drive...