Sorry it took me so long to get back to you. I was in Vegas for CES, and was just swamped with meetings the entire time. I'm finally home.
SXSW is huge. If you've never been, you can only imagine how it takes over part of the city. I've been to shows that are packed, and I've been to many that I was the only person there. There are the official SXSW showcases, which take place primarily at night, and then there are the unofficial shows, which account for 75% of the weeks entertainment. On a typical SXSW day, I'll leave the house at 10 am, and not get home until 3am. I ride a bike because I'm willing to go out of the way to see a band I'm really interested in. Most people won't. The majority of people never leave the downtown area.
Your band's success at SXSW will depend upon the location of your shows. Shows are all over the city, but 99% of the people won't leave the area in the box below.
If your shows are in (or very near) this area, you can draw people, if it is outside, don't expect many people to come. (There are a few places that are notable exceptions: Emo's, Beauty Ballroom, Spider House, Trailer Space Records, French Legation Museum). The vast majority of people walk, and simply won't venture too far because there are plenty of other options close by. The other challenge is what the venue is offering. Some bars have a good reputation and people will go there all day and not leave. Places that aren't bars can't sell liquor, so they give it away. Free food and other giveaways are common as well.
What is your press packet like? Good promo shots? Are the ladies attractive? Guys will go see bands just because they have attractive girls. That can definitely be an advantage for you.
My advice: Don't come unless you have more that one show lined up at a club or venue in the area outlined above. Your best bets are parties that are giving away food and drink. Don't try to crash it with pop up shows, or get shows once you're here unless you have quality shows already lined up.