I've got almost every bolt on and the X-pipe has my vote for best bang for the buck. I found my Bassani catted x-pipe used for $200. You can find a new off-road one for less.
Though... (I know, double post, stfu already) best 1/4 mile improvement was definitely my suspension... find an adjustable shock/strut with some Eibach pro-kits (total price tag for the D-specs and Pro Kit for me = $729.00) and your 60 foots and overall times will drop like mad... my best 60' went from a 2.20 to a 1.97 and my best 1/4 time went from a 14.173 to a 13.805... consitently running 14.22s to now 13.90s in a convertible.