...I did the same mistake. I went and bought slip-on ... which absolutely did nothing to add some performance benifit to the busa. I got so desperate to increase some power on the busa, that I ended up ordering a full system again. Believe me when I say it... dont waste your money on bolt-on's. I mean... goot bolts dont come cheap! If you were to but a yosh carbon fiber bolt-on, you will end up spending a lil more than 800$ .... on the other hand you can get a full exhaust system for about 1100 - 1200 $$, and it adds a performance edge to your bike.
I suggest you wait ... save up some dough and go for the full system at the first shot.
Think about it... you wont regret putting in a full exhaust system!