Re: stock pipes, or yoshimura?
If you are looking to get more power out of the bike you will certainly like the pipe, but you will also need to jet the bike to match the new airflow. It will give you a small bump in power, nothing major, but on such a small bike every little bit helps =) Most people don't hold on to a 250 long enough to make it worth putting money into, but I would say a louder pipe makes you more noticeable in traffic to go along with more power. Definitely check with the inspectors that would be checking over the bike or check the regulations carefully to make sure that the pipe would be a modification that you would be allowed to do without getting yourself into trouble.
/**Serenity**\ 2002 Honda CBR954RR Fireblade
"But risks must be taken, because the greatest risk in life is to risk nothing. Only the person who risks is truly free."