What bitrate are your blu-ray MKVs? 15-20Mbit is around the most I could push to the xbox (sometimes higher, but rare exceptions). Though what helped me most was having a gigabit switch that supports flowcontrol (hp 1810g-24); have you tried configuring this on your htpc NIC?
When you create your MKVs, make sure you have header-compresson set to "none" for each of the tracks (don't leave it blank) - this was a source of issue when I first started mkvs with the xbox as well.
Are you running Win 7 64bit or 32bit? If 64bit, make sure you unchceked Disable DIVX Foundation in the 64bit shark007 app, not the 32bit. (side note - I find that label combined with the checkbox unnecessarily confusing).
If you're still having issues, you may want to post in Shark's forum - he's pretty good at answering questions.