Damn Straight
Yeah Pirahnas can be used at tourneys, the DUPS (my team) borrowed site pirahnas from our sponsor, 6th Element Paintball, to play in the series 2001 (in the UK). We (as total nebies) won some games, and by the end of the season we had mostly bought our own kit (though mine went down and I played most of Simply The Best Speedball with a pirahna, which was sweet all day), but the reason we were winning more games (we finished 4th in our division) was probably at least as much to do with experience as the kit we were using.
As a starter tourney marker you can feel bit out-gunned, but until you have got used to playing tourney then ask to try out other people's kit and decide what you want. Second hand deals are often pretty good, but if you end up wanting a brand new custom angel, then you'll have had time to save up before having to fork out, and it may save you the expense of an intermediate marker.
Having said that... don't let anyone sucker you with the "Its the man not the marker" if you are not the best player in the world, its a hell of a lot easier to win with superior kit.