You say that you use ffdshow for resize and sharpening yet you have no processing configured (no resize and sharpening)??
Anyway what I think happens is that when adding any filter after PowerDVD you disable its HW acceleration (same thing would happen if you add Vobsub).
The solution for different settings in ffdshow is profiles. You should make MP load ffdshow every time but configure ffdshow to use different profiles depending on the material played. In ffdshow Video decoder configuration/Profiles /Preset settings create a new profile. You can configure different ffdshow settings for the two cases. Next you need to enable Automatic preset loading and then open Preset autoload conditions with the new profile selected (use the Default profile for HD and the new profile for SD for example). Now you need to find some differences between the two video materials.
I for example use my Default profile with deinterlacing disabled for HD material. My "Deinterlace" profile loads only when ffdshow FOURCC match "NV12" which only happens for SD MPEG2 material for me. This might not be applicable in your case.
You can see which profile is loaded by simply right click ffdshow's notification area icon.
Good luck
