I've never worked when the summer district convention is here, but our restaurant is pretty close. I don't think we've ever given just one table for 30 of them though; they actually do divy up into groups of 3-5. I've never heard anything negative (not anything more than any other kind of convention group, at least). You take your chances with a big group, regardless of who they are. I have served them in small groups through the week, but I haven't really noticed anything out of the ordinary in regards to the tip.
If we get Americans though, I find that they tend to tip less - or in coin (or a couple dollar bills), thinking that their coins are so much more valuable than Canadian currency ;)
Tammy