trophyhunt,
Take this information with a grain of salt as I have not yet verified it. Here is what I found out when looking into this same issue on my standard BlackDiamond last year. I was toying with going to a peep sight so I was reading here and there on this sight about just the problem you are describing. I called Thompson to check out some information I read about there being 2 front sights that fit these rifles.
I was told that it was true..there is a part #61006255 with a height of .470 and part #39201280 with a height of .340. My understanding is the first site is the norm, the second is for a different rifle that does fit the mounting.
As it turned out, I did end up going with a Williams peep on the back, not 100% sure on the model. but it was listed for my BD. As it turned out I did not have to make any modifications, but if I remember correctly the peep sight can be located in one of two locations on the standard BD, not sure if it the same with the XR. I had to play with that location a bit to see which worked best for me.
I just realized the other day when clecking my stuff that my front sight on the BD is busted, I can probably just replace the fiber, but I think I'm going to go ahead and pick up one of each of the above to play with. I missed them today by an hour, so it looks like tomorrow on that.