You know HTC's answer to high end video playback: these are business devices. But they have done graphics acceleration (TF3D needs it), and that helps considerably, given the device's high resolution. It's been several months since the folks at Core Player released a new version for WinMo, and it would be interesting to see what they could do (if anything) with the Diamond2 and Touch Pro2 in terms of tweaks for optimal playback.
At default settings (QTv Display/high video quality), full screen, Core Player benchmarked an MPEG4 video (H.264, 48kHz AAC stereo-- basically iPhone optimal settings) 480 x 368 resolution, 856kbps at 87.26%. As mentioned in the review, the phone likes it if you keep it at 600kbps or under, hence the 87% figure.
A QVGA WMV file encoded at 406kbps played at 255%.
The Chosen, our old standby MPEG benchmark file of old played at 254%. It's QVGA and encoded at 311kbit/s.