Sounds like you're working for VM so not entirely impartial
TBH. And I can show that XBL traffic is throttled and/or speedtests and HTTP traffic is given priority (for me anyway), probably to mislead customers into thinking they are getting the advertised speed for everything. Use Google, I'm far from the only one to have observed this but of course it's not something a novice would be able to measure. If the graph I posted on this thread isn't enough I can do some more for you, but it will be more of the same really. If you prove me wrong (issue in my area for instance), I'll happily edit these posts but as it stands what I say is entirely true.
I will say I haven't done any tests on my mate's 50Mbps connection like I've done on mine (it does seem 50Mbps is less harshly throttled) but it's quite pathetic that he couldn't host a game properly and I concluded it was speed limiting because smaller games worked fine.
I was reading a thread over at the VM forums a while ago when I was searching for information regarding this, someone found the same as me, a VM employee admitted it and said something along the lines of 'we do not support online gaming on our network so there's nothing we can do about it'. The thread seems to have been removed but I'll see if I can find it in the archives or whatever.
Oh and Hexus have also reported on it:
http://gaming.hexus.net/content/item.php?item=30147