There's not really a way to send an unprocessed/unscaled image to an external video processor right now.
1. All commercial video is distributed in a 4:2:0 format right now, and HDMI prior to 2.0 only supports a minimum of 4:2:2, so chroma upscaling must be performed in the player.
2. I'm not sure that Media Center has the option to disable aspect ratio correction for SD sources. (players like MPC-HC do offer this) This is a problem for that kind of comparison because video is stored as 720x480 with non-square pixels, and then corrected to either 4:3 or 16:9.
3. HTPC sources work with RGB internally, but video is stored as YUV. This means they have to convert from YUV to RGB to display an image - if you then choose to output YCbCr from the HTPC, it goes through a further conversion step. madVR does high quality YUV > RGB conversion, but your video card handles the output conversion if you are not using RGB, and it may not be done with much precision. Stand-alone players will likely avoid this RGB step and remain in YUV for processing the image.
VideoClock processes the audio to keep it in sync with the video, preventing dropped frames. It's not going to affect image quality.
There should not be any sharpening happening with Media Center's video playback.
When processing video, you really want to have access to the original unscaled 4:2:0 source data, which means that you need to process it inside the player rather than an external box. Devices like the Radiance are more suited to live TV (where you don't have the option of a high end "player") than DVDs.
I think you will find madVR to be capable of image quality equal to or better than the Radiance offers.
madVR goes straight from the source 4:2:0 YUV data to RGB (4:4:4) in one step whereas the Radiance only has access to a pre-processed 4:2:2 image, and if you send it 4:4:4 data, it is downsampled to 4:2:2 internally.