the cherry bomb type are very effective imo (i have 3 on mine 
)
where-as the oval type silencers can have a poor flow, where the pipe is blanked off (arrowed) and the gasses have to fight out of the perforations, across the box, then into the other perforations into the tailpipe fwn
http://www.micra.org.uk/attachment.php?attachmentid=13826&stc=1&d=1258989195
