Let me start by saying I'm appalled at WinMo's inability to play it's own video format (wmv) even at its crappiest quality settings. Luckily there are mp4's.
I've found that Juicy's 4.3 or 4.4 has the best video performance and generally plays everything. My fav to watch is:
CoreAVC h.264
480x270 (widescreen)
@ ~23fps
Bitrate ~1Mbps
If you want higher res like 640x360
You will probably want to drop the framefrate to 15-20.
I've also seen good results with Flv h.263 (youtube/google video files)
http://msdn.microsoft.com/en-us/library/aa451351.aspx
These are the only "tweaks" that you can do without using new drivers.
Plus, most of the time, they are already set at the best options for the devices abilities.