I am having the EXACT SAME PROBLEM!
Whew! I thought it was just me
Anyway, I am using the Intec HD AV pack with my ORIGINAL xbox (not 360) with my LG DLP HDTV and whenever I choose HD resolutions in XBMC, I get these shadows that you're describing. I don't play xbox games anymore, I only use my xbox1 for XBMC so I can't comment on xbox games. But these shadows are driving me NUTS. (YES, I have the switch over to HD on the AV pack)
You can try my workaround ...
This may sound strange..but try plugging an S-VIDEO cable into the HD pack alongside your component cables and then plug the other end of the S-VIDEO cable into any S-VIDEO port on your TV.
Let me know what happens.
I had this working just fine up until I purchased a component input switcher so that I could use both my Wii and my Xbox1 through the same component input on my TV. All was GREAT until the screen turns completely white, then the signal gets garbled and the screen flickers black and then goes back to normal.
When I plug my xbox directly into my TV (remember..using BOTH component AND s-video cables) it works perfectly and looks beautiful in 1080i.