We usually play Team Slayer in the evening when there are around 30-40k players and we play on separate accounts with a few hundred games played each. I should clarify that it used to work flawlessly for us but lately we've been having a lot of trouble with it. It does work *sometimes* and when it works, it works great and consecutively but when it doesn't work, it just doesn't. It's a crap shoot.
Sorry, I left out a monumentally important detail. Up until recently, we've been playing split screen but now we're playing on two separate Xboxes on the same network. It seems like this is when the inconsistency in matchmaking began.